Modification of MMDS Converters

for Radio Amateur use

The MMDS (Multipoint Microwave Distribution System) converters are used for the distribution of video cable signals to open areas outside of the urban centers.
They work in S band, and convert  the range of  frequencies that goes from 2500 MHz to 2686 MHz to a FI of 222 to  408 MHz, which correspond to the cable TV channels from 23 to the 55 approximately.
These converters, are made by different suppliers, and in general they consist of an input filter, a low noise amplifier, a second filter, a mixer, a local oscillator and a iIntermediate frequency amplifier. The first photo shows a California Amplifier converter at the front and one of taiwanese manufacturing in the back, with the original dipole replaced by a 3 turns helix.
As you can imagine, the modification tasks require of certain experience, and they are not easily attainable by a beginner, in addition to require of at least a multimeter and a frequency meter for the adjustments, you will also have to  take all the necessary precautions to avoid damage to the components, specially the GASFETs and the small surface mounted devices.
I modified a pair of these converters, one to convert from 2400 MHz to 144 MHz to be used in the reception of ham radio satellites in that band, and the other to convert from 1691MHz to 137,5 MHz to receive GOES weather images  with a NOAA images receiver.
The first stage to modify is the local oscillator, which works originally in 2278 MHz, to a frequency that will provide the wished conversion. In the case of GOES reception, the new frequency would be 1828,5 MHz.
The local oscillator is normally a PLL circuit, the VCO frequency is divided by 256, so that the reference oscillator works at 8,898 MHz, therefore, to take the oscillator to the new frequency we must replace the crystal by one of 1828,5/256 = 7,14257 MHz. we must also add capacity to the VCO's tuned circuit  until the PLL is locked in the new frequency. This can be verified when the correction voltage applied to the VCO is approximately 2 V, this voltage can be measured in the test point in the printed circuit (see photo 2 - "punto de prueba").
The capacitor to be added to the VCO's tuned circuit  is really a piece of about 15 mm of 2 adjacent wires from a flat cable of the ones used in computers to interconnect discs and diskette drives (to see photo 2).
The oscillator's frequency can be adjusted with the tunning trimmer near the crystal, this ismeasured in the input to the phase comparator (see figure). In the same way the frequency of the VCO can be measured, after going through the frequency divisor, in the other input of the phase comparator, in both cases we should read 7,142578 MHz.
Once  the local oscillator is modified, we must remove the two filters located before and after the low noise amplifier, at the other side of the converter. To do this it is necessary to remove the 6 screws that hold the filter covers, remove the comb looking filters, and to replace them by a piece of wire (to see photo 3).
If the converter will be used to receive 2400 MHz, the modification is simpler, since the local oscillator frequency will be moved a few MHz, for this reason, merely replacing the crystal and adjusting the tunning trimmer, will ensure that the PLL is locked to the new frequency, and the filters can be fit, adjusting them to  2400 MHz, with the 3 "Torx"screws  that hold the filter covers.
If the converters were to be used with scanner type receivers, which can tune all the VHF and UHF range, it is not necessary to modify the local oscillator, we must calculate the difference between the frequency  we want to receive and the 2278 MHz local oscillator's frequency to know which frequency we need to tune in the receiver, and it is necessary only to remove the converter's filters or to modify them for the frequency to receive.

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email: lu7dsu@amsat.org