아마추어무선사들이 사용하는 VoIP 네트워크중

대표적인, EchoLink 의COM 기반 API 설명서.

MS HELP 포멧 파일.

수신단이 IF 트랜스포머에 의한 주파수 낮춤을 하지 않고 송신단이 체배기등을

사용하지 않는지는 실물을 뜯어 보지 못했고, 웹사이트에서도 회로도를 제공하지

않아서 알수는 없지만 어쨌던 PC 용 소프트웨어와 DSP (디지탈 신호 처리 칩) 로

구성되어 실제 운용이 가능한 SDR (Software Difined Radio) 무전기이다.

위에 말한 IF 단이 존재하는 SDR 비슷한 무전기는 기존에도 많았지만, 완전히 SDR

개념으로 출시된 제품은 처음이라고 하는데… 확인한 바 없다. 어쨌던 미국 아마추어

무선 연맹이 발간하는 월간지 QST 2005년 4월호 리뷰에선 아직까지 평범한

무선사들을 위한 것은 아니다.라고 평가했으니, 아마 얼리어뎁터급 무선사들이

관심을 많이 가지리라…

1w 송신부를 가진 제품이 899불, 100w 송신부 옵션을 포함한 것은 1375불,

내장 자동 안테나 튜너를 230불에 추가 할수 있으며, 144MHz 용 트랜스버터도 190불에

옵션으로 제공된다. 한국까지 항공소포요금은 52불이다. 아… 지름신이 나의 어깨를… ^^;

자세한 정보는 플렉스 래디오http://www.flex-radio.com 에서 볼 수 있다.

클릭하면 제대로 나옵니다.

현충일 밤, 나의 Primary Radio 인 FT-857D 가 개조를 위해 배를 갈랐다.

배를 가른 후 작업전 모습이다. 근데 공구가 고장나서 아직 수술이 안 끝났다. 불쌍한넘. –;

SINGLE-SIDEBAND

You know from studying the single-sideband transmitter material in this chapter you may transmit only one sideband of an AM signal and retain the information transmitted. Now you will see how a single-sideband signal is received.

Advantages

Figure 2-11 illustrates the transmitted signal for both AM and ssb. Ssb communications has several advantages. When you eliminate the carrier and one sideband, all of the transmitted power is concentrated in the other sideband. Also, an ssb signal occupies a smaller portion of the frequency spectrum in comparison to the AM signal. This gives us two advantages, narrower receiver bandpass and the ability to place more signals in a small portion of the frequency spectrum.

Figure 2-11.—Comparison of AM and SSB transmitted signals.

SSB communications systems have some drawbacks. The process of producing an ssb signal is somewhat more complicated than simple amplitude modulation, and frequency stability is much more critical in ssb communication. While we don’t have the annoyance of heterodyning from adjacent signals, a weak ssb signal is sometimes completely masked or hidden from the receiving station by a stronger signal. Also, a carrier of proper frequency and amplitude must be reinserted at the receiver because of the direct relationship between the carrier and sidebands.

Figure 2-12 is a block diagram of a basic ssb receiver. It is not significantly different from a conventional superheterodyne AM receiver. However, a special type of detector and a carrier reinsertion oscillator must be used. The carrier reinsertion oscillator must furnish a carrier to the detector circuit. The carrier must be at a frequency which corresponds almost exactly to the position of the carrier used in producing the original signal.

Figure 2-12.—Basic ssb receiver.

Rf amplifier sections of ssb receivers serve several purposes. Ssb signals may exist in a small portion of the frequency spectrum; therefore, filters are used to supply the selectivity necessary to adequately receive only one of them. These filters help you to reject noise and other interference.

Ssb receiver oscillators must be extremely stable. In some types of ssb data transmission, a frequency stability of ±2 hertz is required. For simple voice communications, a deviation of ±50 hertz may be tolerable.

These receivers often employ additional circuits that enhance frequency stability, improve image rejection, and provide automatic gain control (agc). However, the circuits contained in this block diagram are in all single-sideband receivers.

Carrier Reinsertion

The need for frequency stability in ssb operations is extremely critical. Even a small deviation from the correct value in local oscillator frequency will cause the IF produced by the mixer to be displaced from its correct value. In AM reception this is not too damaging, since the carrier and sidebands are all present and will all be displaced an equal amount. Therefore, the relative positions of carrier and sidebands will be retained. However, in ssb reception there is no carrier, and only one sideband is present in the incoming signal.

The carrier reinsertion oscillator frequency is set to the IF frequency that would have resulted had the carrier been present. For example, assume that a transmitter with a suppressed carrier frequency of 3 megahertz is radiating an upper sideband signal. Also assume that the intelligence consists of a 1-kilohertz tone. The transmitted sideband frequency will be 3,001 kilohertz. If the receiver has a 500-kilohertz IF, the correct local oscillator frequency is 3,500 kilohertz. The output of the mixer to the IF stages will be the difference frequency, 499 kilohertz. Therefore, the carrier reinsertion oscillator frequency will be 500 kilohertz, which will maintain the frequency relationship of the carrier to the sideband at 1 kilohertz.

Recall that 1 kilohertz is the modulating signal. If the local oscillator frequency should drift to 3,500.5 kilohertz, the IF output of the mixer will become 499.5 kilohertz. The carrier reinsertion oscillator, however, will still be operating at 500 kilohertz. This will result in an incorrect audio output of 500 hertz rather than the correct original 1-kilohertz tone. Suppose the intelligence transmitted was a complex signal, such as speech. You would then find the signal unintelligible because of the displacement of the side frequencies caused by the local oscillator deviation. The local oscillator and carrier reinsertion oscillator must be extremely stable.

Q16.   What two components give a ssb receiver its advantages over an AM superheterodyne receiver?

출처:http://www.tpub.com/content/neets/14189/css/14189_48.htm

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