Synchronous demodulation / detection
!
Today’s radio receivers offer very high levels of performance and boast many facilities.
Many radio receivers incorporate memories, phase locked loops, direct digital synthesis,
digital signal processing and much more. One facility that can be very useful on the short
wave bands is synchronous detection or synchronous demodulation as this can give
much improved performance for receiving amplitude modulation (AM) transmissions.
Unfortunately little is written about this form of modulation, and often it is a matter of
accepting that it must be better than any normal options because it is included as a
feature in the receiver specification.
!
Synchronous detection is used for the detection or demodulation of amplitude modulation
(AM). This form of modulation is still widely used for broadcasting on the long, medium
and short wave bands despite the fact that there are more efficient forms of modulation
that can be used today. The main reason for its use nowadays is that it is very well
established, and there are many millions of AM receivers around the world today.
!
In any receiver a key element is the detector. Its purpose is to remove the modulation
from the carrier to give the audio frequency representation of the signal. This can be
amplified by the audio amplifier ready to be converted into audible sound by headphones
or a loudspeaker. Many receivers still use what is termed an envelope detector using a
semiconductor diode for demodulating AM. These detectors have a number of
disadvantages. The main one is that they are not particularly linear and distortion levels
may be high. Additionally their noise performance is not particularly good at low signal
levels.
!
These detectors also do not perform very well when the signal undergoes selective
fading as often occurs on the short wave bands. An AM signal contains two sidebands
and the carrier. For the signal to be demodulated correctly the carrier should be present
at the required level. It can be seen that the signal covers a definite bandwidth, and the
effects of fading may result in the carrier and possibly one of the sidebands being
reduced in level. If this occurs then the received signal appears to be over-modulated
with the result that distortion occurs in the demodulation process.
!

The spectrum of an amplitude modulated signal
!
Diode envelope detector
In virtually every receiver a simple diode envelope detector is used. These circuits have
the advantage that they are very simple and give adequate performance in many
applications.
The circuit of a typical detector is shown in Figure 2. Here the diode first rectifies the
signal to leave only the positive or negative going side of the signal, and then a capacitor
removes any of the remaining radio frequency components to leave the demodulated
audio signal. Unfortunately diodes are not totally linear and this is the cause of the
distortion.
!

An envelope detector for AM signals
!
What is synchronous demodulation
Signals can be demodulated using a system known as synchronous detection or
demodulation. This is far superior to diode or envelope detection, but requires more
circuitry. Here a signal on exactly the same frequency as the carrier is mixed with the
incoming signal as shown in Figure 2. This has the effect of converting the frequency of
the signal directly down to audio frequencies where the sidebands appear as the
required audio signals in the audio frequency band.
!
The crucial part of the synchronous detector is in the production a local oscillator signal
on exactly the same frequency as the carrier. Although it is possible to receive an AM
signal without the local oscillator frequency on exactly the same frequency as the carrier
this is the same as using the BFO in a receiver to resolve the signal. If the BFO is not
exactly on the same frequency as the carrier then the resultant audio is not very good.
!

Synchronous demodulation
!
Fortunately this is not too difficult to achieve and although there are a number of ways of
achieving this the most commonly used method is to pass some of the signal into a high
gain limiting amplifier. The gain of the amplifier is such that it limits, and thereby
removing all the modulation. This leaves a signal consisting only of the carrier and this
can be used as the local oscillator signal in the mixer as shown in Fig. 4. This is most
convenient, cheapest and certainly the most elegant method of producing synchronous
demodulation.
!

A synchronous detector using a high gain-limiting amplifier to extract the carrier
!
Advantages of synchronous detection
A synchronous detector is more expensive to make than an ordinary diode detector when
discrete components are used, although with integrated circuits being found in many
receivers today there is little or no noticeable cost associated with its use as the circuitry
is often included as part of an overall receiver IC.
!
Synchronous detectors are used because they have several advantages over ordinary
diode detectors. Firstly the level of distortion is less. This can be an advantage if a better
level of quality is required but for many communications receivers this might not be a
problem. Instead the main advantages lie in their ability to improve reception under
adverse conditions, especially when selective fading occurs or when signal levels are low.
!
Under conditions when the carrier level is reduced by selective fading, the receiver is
able to re-insert its own signal on the carrier frequency ensuring that the effects of
selective fading are removed. As a result the effects of selective fading can be removed
to greatly enhance reception.
!
The other advantage is an improved signal to noise ratio at low signal levels. As the
demodulator is what is termed a coherent modulator it only sees the components of
noise that are in phase with the local oscillator. Consequently the noise level is reduced
and the signal to noise ratio is improved.
!
Unfortunately synchronous detectors are only used in a limited number of receivers
because of their increased complexity. Where they are used a noticeable improvement in
receiver performance is seen and when choosing a receiver that will be used for short
wave broadcast reception it is worth considering whether a synchronous detector is one
of the facilities that is required.

!
출처:http://www.radio-electronics.com/info/receivers/synchdet/sync_det.php

Synchronous Vs Envelope Detection Updated 10/05/01

Some demodulators provide a choice between envelope and synchronous detection modes. When running the FCC Proof of Performance tests, its important to use the proper detection mode. Especially when measuring differential phase. Let’s take a look at the two modes and see what they have to offer.

Envelope Detectors

Envelope detectors are usually considered to be the simplest form of detector. In fact, they can be quite complicated or they can be as simple as a diode and a low-pass filter. The performance of an envelope detector, when used to demodulate a television signal, is limited by the relative amplitudes of the signal being demodulated (bigger is better), and an effect called quadrature distortion. Quadrature distortion is a result of the asymmetry of our television signal. It’s vestigial sideband coming from the modulator or transmitter and nearly single sideband within the television’s IF circuit. One nice thing about envelope detectors (in addition to their low cost) is that they are not sensitive to the phase of the visual carrier. This will become important when we look at differential phase tests.

Synchronous Detectors

Synchronous detectors are considerably more complex than simple envelope detectors. They consist of phase locked loop and multiplier circuits. Demodulation is performed by multiplying the modulated carrier by a sine wave that is phase locked to the incoming carrier. Synchronous detectors are a subset of "product" detectors. If you are an amateur radio operator, you may have listened to single sideband suppressed carrier signals using receivers with a BFO for re-inserting the carrier and envelope detection; and other receivers with real product detectors. SSB signals sound much better using product detectors.

The advantage of synchronous detection is that it causes less distortion than envelope detection and works well with single sideband signals. It is the preferred detection method for most tests. Another characteristic of synchronous detectors is that they are phase sensitive. The amplitude of the demodulated signal is a function of the relative phases of the incoming carrier and the carrier generated inside the receiver. In the extreme case, if the phase of the modulated carrier and the regenerated carrier in the demod is 90 degrees (they are in quadrature) the detector output would be zero!

출처:http://www.tvms.net/Tech_Articles/Synchronous_vs_Envelope_Detection.htm

HONGIK UNIVERSITY Telematique Lab.

4-1 단측대역 특성

전력 분배(Power Distribution)

피크 포락선 전력(Peak Envelope Power) : 단측파대 송신기의 출력 전력을 재는 데 사용된 수단.

측대역 전송의 형태(Types of Sideband Transmission)

지시 반송파(Pilot Carrier) : SSB에서 억압 반송파, 반송파는 낮은 레벨로 감소되나 완전히 제거되지는 않는다.

쌍측대역 억제 반송파(Twin-Sideband Suppressed Carrier) : 두 개의 독립적인 측대역의 전송. 다른 정보를 포함하고 원하는 레벨까지 억압된 반송파를 가짐.

독립 측대역 전송(Independent Sideband Transmission) : 쌍측대역 억압 반송파 전송에 적합한 또 다른 이름.

4-2 단측대역 발생 : 평형 변조기.

평형 변조기(Balanced Modulator) : 반송파가 제거된 상태로 양 대역을 생성하기 위해 반송파와 함께 정보를 혼합한 변조단.

이중 측대역 억제 반송파(Double sideband Suppressed Carrier) : 평형 변조기의 출력 신호.

평형 환 변조기(Balanced Ring Modulator) : 환 구성에서 4개의 정합된 다이오드를 연결하는 평형 변조 설계.

환 변조기(Ring Modulator) : 평형 환 변조기에 적합한 또 다른 이름.

격자 변조기(Lattice Modulator) : 평형 환 변조기에 적합한 또 다른 이름.

LIC 평형 변조기(LIC Balanced Modulator)

그림 4-1 평형 환 변조기

4-3 SSB 필터

그림 4-3 측대역 억제

표면 음향파(Surface Acoustic Wave) : TV와 레이더(Radar) 응용에 종종 사용된 아주 높은 Q-필터.

수정 필터(Crystal Filters)

그림 4-4 수정 등가회로(a)와 필터(b)

위상 커패시터(Phasing Capacitor) : 180°위상차에 의해서 또 다른 커패시턴스의 효과를 제거

제거 노치(Rejection Notch) : 필터에 의해 감소된 주파수의 좁은 범위.

세라믹 필터(Ceramic Filters)

그림 4-5 세라믹 필터와 응답곡선

세라믹 필터

형성인자(Shape Factor) : 높은 Q대역 통과 필터의 60dB와 6dB대역의 비율

피크 대 벨리비(Peak-to-Valley Ratio) : 리플 진폭에 적합한 또 다른 이름.

리플 진폭(Ripple Amplitude) : 6dB대역 이내에서 높은 대역 통과 필터의 감소에서의 변화.

그림 4-6 기계적 필터

기계적 필터(Mechanical Filters)

그림 4-7 기계적 필터의 전기적 분석

4-4 SSB 송신기

필터 방법(Filter Method)

변환 주파수(Conversion Frequency) : 평형 변조기에서 반송파에 적합한 또 다른 이름.

그림 4-8 SSB 전송기 블럭도

SSB 발생기 필터(Filter SSB Generator)

수정-격자 필터(Crystal-Lattice Filter) : 적어도 두 개 수정을 포함하는데, 일반적으로 4개의 수정을 포함하는 필터.

연속파(Continuous Wave) : 라디오 전송장치에서 발진기에 의해 생성된 비제동 정현파 형태.

위상방법(Phase Method)

그림 4-10 위상 천이 SSB발생기

ACSSB 시스템(ACSSB Systems)

압신기(Compandor) : 압축/확장 ; 보다 좋은 잡음 수행을 제공하기 위해 전송 장치에서 가변 이득회로는 낮은 레벨신호를 적합한 이득을 증가시킴 ; 수신자에서 상보회로는 원래 신호를 재생하기 위해 진행을 반대로 한다.

그림 4-12 ACSSB 신호

4-5 SSB 복조

파형(Waveforms)

그림 4-14 AM, DSB, SSB파형 정현곡선 변조신호.

혼합기 SSB복조기(Mixer SSB Demodulator)

그림 4-15 SSB 복조기로 사용되는 혼합기

BFO 표동 효과(BFO Drift Effect)

곱 검파기(Product Detector)

곱 검파기(Product Detector) : 단측 측파대 신호에서 정보를 재생하기 위해 평형 변조기를 사용.

4-6 SSB 수신기

그림 4-17 SSB 수신기 블록도

기본 SSB 수신기(Basic SSB Receiver)

Butterworth 필터(Butterworth Filter) : LC필터의 상수-k 형태.

출처:http://shinan.hongik.ac.kr/%7EMmcl/workshop/ch04/chapter%204%20Single%20Sideband%20Communications.htm

반송파 및 한쪽의 측파대를 제거하고 단일측파대만 사용하여 신호파를 전송하는 방식.

단측파대전송은 SSB(single side band)전송방식이라고도 한다. 진폭변조(振幅變調)된 신호파를 반송파(搬送波)로 변조시키면 피변조파(被變調波)의 스펙트럼은 반송주파수를 대칭축으로 2개의 상 ·하측파대(上下側波帶)가 된다. 이 피변조파 중 상측파대는 신호파의 스펙트럼을 그대로 높은 주파수대로 이동시킨 것이고 하측파대는 신호파의 스펙트럼을 반전하여 낮은 주파수대로 이동시킨 것이다.

2개의 측파대는 반송파를 중심으로 대칭을 이루고 있기 때문에 한쪽만을 전송해도 수신측에서 재생할 수가 있다. SSB파를 복조(復調)하는 데는 일반적인 진폭변조에서 사용하는 정류검파(整流檢波)로는 불가능하고, 수신측에서 국부발진파(局部發振波)를 가해서 헤테로다인(Heterodyne)검파를 사용하면 변조신호파만의 출력을 얻을 수 있다. 측파대를 발생시키는 방법은 먼저 평형변조기(平衡變調器)를 사용하여 반송파 성분을 억압하고 그 다음에 필터를 사용하여 필요로 하는 측파대의 스펙트럼만을 분리 ·선택한다.

단측파대 전송의 이점은 첫째, 점유주파수폭(占有周波數幅)이 양측파대 전송시의 1/2이다. 둘째, 반송파 및 한쪽 측파대를 보내지 않으므로 증폭기 등의 소요전력을 감소시킬 수 있다. 셋째, 대역폭이 좁으므로 잡음이 감소되어 신호대잡음비(信號對雜音比)가 향상되어 수신감도(受信感度)가 개선된다. 넷째, 반송파에 의한 각 통화로 사이의 혼변조(混變調)를 없앨 수 있다.

출처: http://100.naver.com/100.php?id=42927 (네이버백과사전)

Beat-Frequency Oscillator

The beat-frequency oscillator (bfo) is necessary when you want to receive cw signals. Cw signals arenot modulated with an audio component, you remember, so we must provide one. The action of the rfamplifier, mixer, local oscillator, and IF amplifier is the same for both cw and AM; but the cw signalreaches the detector as a single frequency signal with no sideband components. To produce an af output,you must heterodyne (beat) any cw signal with an rf signal of the proper frequency. This separate signal isobtained from an oscillator known as a beat-frequency oscillator.

Figure 2-20 is a block diagram of a superheterodyne receiver capable of receiving and demodulatinga cw signal. The bfo heterodynes at the detector and produces an af output. The detector (second detector)is used primarily because the mixer (first detector) is normally used as the source of agc.

Figure 2-20.—Placement of the beat frequency oscillator.

If the intermediate frequency is 455 kilohertz and the bfo is tuned to 456 kilohertz or 454 kilohertz,the difference frequency of 1 kilohertz is heard in the output. Generally, you will tune the bfo from thefront panel of a receiver. When you vary the bfo control, you are varying the output frequency of the bfoand will hear changes in the tone of the output audio signal.

출시:http://www.tpub.com/content/neets/14189/css/14189_57.htm

주파수의 차가 근소한 2개의 파동이 간섭을 일으켜, 두 주파수의 차에 따라서 진폭이 주기적으로 변하는 합성파(合成波)가 이루는 현상.

이를테면 동시에 전해 오는 두 음이 규칙적으로 강해졌다 약해졌다 하는 현상을 말한다. 두 파동체뿐만 아니라 한 파동체에서도 진동수가 부분적으로 다를 때에는 맥놀이현상이 일어나는 경우가 있다. 예를 들면 범종(梵鐘)의 은은한 여운 같은 것인데, 이것은 재질이나 두께의 불균일, 모양의 비대칭성 등이 원인이 되어 종의 각 부분에서 다른 진동수의 소리가 나오기 때문에 생기는 맥놀이의 일종이다. 단 소리가 똑똑히 들릴 경우의 맥놀이주파수는 6∼7Hz 이하일 때이며, 그 이상이면 소리가 흐려 분간하기 어렵다. 또 두 주파수가 같으면 맥놀이는 사라진다. 이 현상은 이미 알고 있는 주파수에 의해 다른 미지의 주파수를 알아내는 데 이용된다. 전기적인 파동일 경우에는 이 밖에 주파수의 변환에도 이용되는데, 라디오의 헤테로다인 방식이나 수퍼헤테로다인 방식 등의 수신원리는 이를 응용한 것이다.

헤테로다인 [ heterodyne ] – 네이버 용어사전 매스컴용어
 
다른 힘 또는 두 가지 힘의 조합이라는 뜻. 두 개의 또 다른 주파수를 만들어 내기 위해 두 개의 다른 주파수를 결합시킴. 라디오 수신에 중요한 개선이 있게 만든 기술로 페센덴(Reginald Fessenden)에 의해 발견되었다. 비트(beat)라고도 불린다. 무선공학에서는 두 진동 전류가 생기는 비트 현상을 일반적으로 헤테로다인이라고 한다.

헤테로다인 [ heterodyne ] – 네이버 용어사전 IT용어 전파
 
국부 발진 신호를 가함으로써 입력 신호 주파수가 변환되어, 원신호와 같은 변조 정보를 갖는 주파수가 원신호와 국부 발진 주파수의 합 또는 차의 어느 한쪽이 되는 출력을 생성시키는 주파수 변조기 내의 과정.

헤테로다인 검파 [ -檢波, heterodyne detection ] – 네이버 용어사전 IT용어 기초
 
주파수가 낮을수록 선택도가 양호해지는 성질을 이용하여 수신 전파의 주파수를 더욱 낮게 하여 중간 주파수로 변환시킨 다음에 검파하는 방법. 검파관과 국부 발진관을 겸하는 것을 자려(自勵) 헤테로다인 방식, 그렇지 않은 것을 타려(他勵) 헤테로다인 방식이라고 한다.

비트 [ beat ] – 네이버 용어사전 매스컴용어
 
①연기자의 화술이나 연기를 순간적으로 멈추고 쉬라는 표시로 촬영대본 상의 대사 중간에 삽입해 놓은 지시어.②주파수가 조금 다른 두 신호간의 주기적인 증감과 상쇄작용으로 생겨난 제3의 주파수를 비트 프리퀀시(beat frequency)라 한다.③기사의 특종. 스쿠프(scoop).

HETERODYNE – 야후 지식 검색

<무선〉헤테로다인(수신파와 국부 발신파 사이에 맥놀이를 일으키는 검파 방법); 헤테로다인(식) 수신기[장치]. 4 ━ 헤테로다인 효과를 낳다. 0 ━ 〔어떤 주파수〕에 다른 주파수를 섞어서 헤테로다인 효과를 낳게 하다. 9 ━ 헤테로다인 수신법의.
 

Ham Radio BFO

Ham Radio (amateur radio) is a popular hobby amongst electronics enthusiasts all over the world. Basically the hobby involves a person in making his own gear consisting of a receiver and transmitter or a transceiver (a receiver and a transmitter in one unit) after procuring a licence from the Ministry of Communications. Home brewing or self construction, an integral part of the hobby, has been sadly neglected in our country, despite the fact that various institutions with governmental help have come into being recently.

Hams aboard can buy the latest transceiver off the shelf at a reasonable price and go on the air immediately. But in India, with a sixty per cent duty involved (now changed?), a commercial transceiver would cost a whopping Rs: 50,000. Hence, it is beyond the reach of an average Indian Ham.

The Indian ham is often handicapped for want of ham gear. To overcome this shortcoming a small receiver and a transmitter can be home brewed with indigenously available components. The total outlay may not exceed a few hundred rupees. Some of you may wonder if this is feasible with out fancy test equipment like oscilloscopes and LC bridges etc. Yes it is possible.

How to get started?

We are all aware that wireless communications means receiving and transmitting messages. We have many modes of HF (high frequency) communications like CW (continuous waves) where the signals are transmitted in the form of morse telegraphy. Generally, radio telephony consists of FM (frequency modulation), AM (amplitude modulation) or SSB (single side band). FM is mostly used for VHF (very high frequency) communications. AM, because of its inherent disadvantages, has become outdated for hams. SSB is the most popular mode used by hams all over the world. It permits them to have long rag-chews (long radio contacts). In addition to this, we have exotic digital modes like packet radio, SSTV (slow scan TV) and satellite communications.

First, let us look into the the earliest and simplest ways of CW and phone communication systems used by most Indian radio amateurs. In our country, we can see much of the amateur activity on 7000 – 7100 KHz (40 meter ham band) and 14000 – 14350 KHz (20 meter ham band). To get acquainted with this hobby, and to sharpen your skills in the communication procedures, you should make it a point to listen to the QSOs (radio contacts between hams) regularly.

The key to the success of an air operation is the possession of a good receiver. A good receiver is mated with a good pair of ears is an unbeatable combination. It is also possible to monitor the conversations between radio hams on an ordinary broadcast receiver. It would be advisable to use a broadcast receiver with three or more bands because it permits better tuning on the small spectrum of ham frequencies.

Only some of these receivers can tune on to 14 MHz (20 meter) band but almost all of them can tune on to 7 MHz (40 meter) band where you find most of the hams from the south chatting away.

Monitoring The Ham Bands

Take a broadcast radio of three bands or more. It can be a valve radio or a two in one.

Next, get an outdoor antenna of any ordinary wire between 20 and 16 SWG with a length of 8 to 15 meters erected at about eight meters or higher above the ground level. The latest transistorised radios may not require this antenna set up as they are very sensitive, but an external antenna is of much help in catching weak stations.

Identifying the band

Now with all set, around 7.30 am tune to the 41 meter band of your radio. Tuning should be done very slowly because of the narrow spectrum of the ham band. A little practice will make you perfect in the art of slow tuning. Now follow the frequencies of the station as given below.

You can easily tune in to Radio Sri Lanka on 7.190 MHz. A little lower down, you can find AIR Madras on 7.160 MHz. Further below you can hear the voice of America right on 7.115 MHz. Just below it is the 7 MHz ham band. Here you will here some AM phone stations saying ‘this is VU2’ followed by their station suffixes. Now you are bang on the ham bands.

In addition to this, you will hear stations sounding like quackling ducks. These are SSB (single side band) phone stations. To make these signals intelligible, you require to add a BFO (beat frequency oscillator). Make a hairline mark on the dial for permanent identification of the ham bands.

Regular broadcast receivers are meant for reception of AM signals. AM signals consists of an RF carrier wave with two side bands (upper side band and lower side band). These side bands are generated when audio signal is modulated with RF carrier. These carry the actual intelligence. Once the side bands are generated with the RF carrier, it is vestigial, consuming most of the power. So while transmitting, the RF carrier is removed from the signal, and the one side band is also removed as both the side bands carry the same information. But at the receiving end the carrier should be reinserted to make the audio intelligible.

In CW mode, RF carrier is transmitted in the form of dots and dashes in morse telegraphy. At the receiving end this incoming carrier should be mixed with another carrier with a difference of 1 KHz to make the signal audible. Such CW and SSB require re insertion of the carrier at the receiving end.

Beat Frequency Oscillator

A simple free running oscillator oscillating at the receiver’s IF (intermediate frequency) beats with the incoming signals in either CW or SSB and the resultant audio frequency goes to the AF amplifier.

The BFO works on any DC supply from 6 to 9 Volts. It can be assembled on any small veroboard. As the circuit is small and simple, no separate PCB design is given. A 2X transistor radio type tuning capacitor can be used to change the frequency back and forth. This unit can be housed in a small battery eliminator box. Power can be borrowed from the broadcast receiver.

BFO’s use

Keep the unit close to the radio. Connect a 30 cm long wire at the output point of the BFO and leave it. This wire will serve as an antenna. RF signals radiated by this wire are sufficient for the radio to resolve CW and SSB signals.

Now switch on the BFO, keep the 2X tuning capacitor at the center. Switch on the radio and adjust the core of the transformer on the BFO with a trimming device until you hear a hissing sound in the radio. Now tune the radio up and down. If the hissing sound appears on the MW and SW you are on the right track in resolving the SSB signals. In this condition, tune in to any station and you will find that the BFO signal is beating with the incoming signal.

Resolving the SSB and CW signals

Switch off the BFO. Tune to the ham band and tune into a SSB signal correctly. Now switch on the BFO and slowly adjust the 2X tuning capacitor until you hear a clear and sharp sound. Leave it in that position. At this stage, you can copy SSB and CW signals, occasionally you may have to change the tuning position. To copy AM stations and regular broadcast stations simply switch off the BFO.

A Amarendra VU2AAP EFY MAY 92

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REGENERATIVE DETECTOR

A simple, one-transistor REGENERATIVE DETECTOR circuit that uses the heterodyning principlefor cw operation is shown in figure 3-4. The circuit can be made to oscillate by increasing the amount ofenergy fed back to the tank circuit from the collector-output circuit (by physically moving tickler coil L2closer to L1 using the regeneration control). This feedback overcomes losses in the base-input circuit andcauses self-oscillations which are controlled by tuning capacitor C1. The received signal from the antennaand the oscillating frequency are both present at the base of transistor Q1. These two frequencies areheterodyned by the nonlinearity of the transistor. The resulting beat frequencies are then rectified by theemitter-base junction and produce a beat note which is amplified in the collector-output circuit. The afcurrents in the collector circuit actuate the phones. The REGENERATIVE DETECTOR (figure 3-4)produces its own oscillations, heterodynes them with an incoming signal, and rectifies or detects them.

Figure 3-4.—Regenerative detector.

The regenerative detector is used to receive short-wave code signals because it is easy to adjust andhas high sensitivity and good selectivity. At high frequencies, the amount of signal detuning necessary toproduce an audio-beat note is a small percentage of the signal frequency and causes no trouble. The use ofthe regenerative detector for low-frequency code reception, however, is usually avoided. At lowfrequencies the detuning required to produce the proper audio-beat frequency is a considerable percentageof the signal frequency. Although this type detector may be used for AM signals, it has high distortion

and is not often used.

Q-7.What simple, one-transistor detector circuit uses the heterodyne principle?

Q-8.What three functions does the transistor in a regenerative detector serve?