Conversion vs Demodulation in STL translators

1. This article is written for the purpose of discussin the advamtages and disadvantages of using the conversion and the demodulation schemes for FM translator operation.

2. With the conversion method the errors of frequency offsets are additive and require high precision quartz references. This is especially true in the 900 MHz range where it is more difficult to mantain the   exat frequency. The demodulation method on the other hand avodis this problem. The transmitter in the 88 to 108 band is much easisr to mantain on a designated frequency.

3. The phase noise of all oscillators is a decreasing function of distance from the carrier; at a few hertz from the carrier this become a significant problem since the synthesizer loop is unable to remove it. If there many conversions in cascade, one can get a phenomenon known as “flicker” i.e. a quivering or trembling of the carrier whitin the 88 – 108 MHz band, that can push the system out of specification. With the demodulation method this effect does not occour since an intrinsec high-pass filter eliminate carrier noise at few hertz from the carrier.

4. All conversion process make use of the mixing of two or more signals of differing frequencies that give rise to an infinite series of resultant frequencies, from which the desired frequency is selected; the relationship is as follows:

Fout=mF1 +/- nF2 (m,m = 0,1,2,3…)

Where Fout are the output frequencies and F1 and F2 are the input frequencies.

It is understood that to select the desired output frequency, it is necessary to use highly selective amplifiers and in any case the output signal spectral purity will always depend on the attenuation of spurious and unwanted signals, decreasing in amplitude as a function of distance from the carrier; moreover certain spurious frequencies may be impossible to eliminate since they fall within the band e.g.:

Intermediate frequency of the receiver = 10.7 MHz

Output frequency of the transmitter = 107 MHz

In this case the local conversion oscillator would operate on a frequency of 96.3 MHz

Fout = mF1 = +/- nF2

Wanted: 107 = 1*96.3 + 1*10.7

Unwanted: = 0*96.3 + 10*10.7

We can see that in tis example, the tenth harmonic of the intermediate frequency falls exactly on the wanted frequency. This cannot be filtered or removed. The output signal will contain the desired signalplus the undesired signal at a lower amplitude modulated at ten times the bandwidth. This way out the specification. Now since m and n can be 0,1,2,3…ect. Is extermely difficukt to analyze all theri possible combinations and to forecast a level of spurios or harmonic of less thann -60 dB

As tge system contains tuned amplifier we are likely to encounter a degradation of the parameters of the system as a fuction of temperature and aging, nor would we overloock the effects of mechanical shock or vibrations caused by the transformer or cooling fan. Given the high q of the tuned circuits, conversion systems are sometimes prone to spurious emissionand must be periodically readjusted with the use of special very expensive test equipement. These spurs may fall into the aircraft or military communications channels. Each output frequency desired for the translator will require a high quality crystal with a long lead time.

In the demodulation system, most of these problems do not exist. All circuit are broadband ando do not require periodic calibration other than possibly the input filter of the receiver. As most system are now sythesized, frequency change is easily done by the user. Factors such as temperature, humidity and aging do not easily affect the quality of the signal and spurious output are not found on the demodulating signal.

5. We can infer that a conversion system does not change the degree of deviation, while the demodulation has a problem in this respect. On a par with the quality of the intermediate frequency filters of booth systems, the first mantains deviation without any problem, however, if the receiver of the second type has a digital pulse counter, there are no problems regarding the variation of modulation level, the demodulator in this case is wideband and does not require calibration, moreocer, noise from this type of demodulator is lower than noise introduced by the oscillator and the S/N ratio of the second system is worse than the first.

6. It is worth nothing here that the critical factor regarding demodulation is the question of output signal quality.

The intermediate frequency pass band of the receiver is less than that of the trasmitted signal, some of the energy of which is attenuated by the receiver’s circuit resulting in significant phase distorsion attributable to the intermediate frequency filters. This ultimately leads tho phase and amplitude distorsion of the signal, (reduced sub-carrier level, reduced stereo separation, increased intermodulation and amplitude distorsion). With the demodulation method, steps are taken to correct these parameters in the base-band (40 to 200KHz).

In conversion system such steps have to be taken at intermediate frequency which leads, with respect to the demodulated frequency, ti greater difficulty as a function of the value of the intermediate frequency; the higher the intermediate frequency, the more difficult it is to cirrect the signal. A receiver with an intermediate frequency of 10.7MHz encouters, at to correct the signal, 50 times more difficulty than a demodulation system. (10MHz/2MHz = 50). For this reason most conversion system manufacturer use very low intermediate frequencies; 2 or 3 MHz. But in spite of this, the difference in the precision of correction is around 10 or 20 times. To reach this level of performance in conversion, tha pass-band of the receiver has to be widened to reduce the amount of correction required. This will however, compromise selectivity and therefore immunity to interference.

The correctioin performed in a demodulation system require relatively less sophisticated laboratory equipment (only low frequency instruments); correction performed on a conversion system require a precision calibration usin complex instruments at R.F. frequencies.

The selectivity of intermediate frequency circuits is not capable of rejecting interfering signals located at 100 to 300KHz from the carrier. In the conversion system those get repeated into the FM transmitter, while in the demodulation system these get rejected by the low pass filter inside the demodulator circuit with a cutoff frequency of 100KHz.

Most manufacturers world wide have abandoned conversion after a few pioneers constructed network using demodulation system, achieving superior results both from the point of view of quality as well as pratically use.