Wednesday, 1 April 2015

Sinadder 3 and Pye SG5U, SG3V signal generators

One of my radio interests is modifying ex-PMR radio equipment for use on the VHF and UHF amateur radio bands.  PMR radios tend to cover quite a wide frequency range, so once they have been programmed or crystalled on the desired frequency they normally need to be aligned to achieve good sensitivity. This can be done while receiving a signal and adjusting the front end for best quieting but better results are achieved using a signal generator and a SINAD meter .

Sinadder 3

SINAD refers to the measurement of 'Signal and Noise plus Distortion' and is a useful way of measuring the performance of a receiver.  The Sinadder 3 manual provides a good description of how this measurement is used to quantify FM receiver performance, but in summary an FM RF signal at the receiver frequency is modulated with a 1KHz audio tone at 5KHz deviation and fed in to the receiver.  The SINAD meter is connected to the audio output of the receiver under test and samples the demodulated 1KHz audio tone.  This can detect the amount of distortion introduced to the 1KHz audio tone bu the receiver. The lower the measured distortion the better the receiver performance.
Typically a measurement of 12dB SINAD is considered a usable received signal, so the aim of alignment via SINAD is to adjust the front end of the receiver to achieve the 12 dB measurement for the minimal level of input signal.

Professional radio service shops tend to use integrated communication test sets or service monitors costing €1,000's for this task but it is possible to pick up older test equipment for a reasonable price which is still very usable for amateur use.

I bought a second hand Helper Instruments Sinadder 3 meter to perform the SINAD measurements but I needed a VHF and UHF signal generator capable of producing an FM modulated output.  My existing signal generator only did AM modulation and more recent signal generators cost hundreds of Euro.
I contemplated trying to build some form of DDS synthesized signal generator using an Arduino and an si5351 device but couldn't see an easy way of frequency modulating it with the required 1KHz tone.  Instead I found a pair of vintage Pye VHF and UHF signal generators going for a good price on eBay.  Shipping from the UK wasn't cheap due to the weight of these devices - way more than the cost of the signal generators themselves , but it was still a good deal for some classic test equipment .

  • The Pye SG3V covers VHF 66MHz to 150MHz
  • The Pye SG5U covers UHF 300MHz to 480MHz

Both are capable of FM modulation with a 1KHz signal with variable deviation from 5KHz to 15KHz . Still useful for wideband amateur equipment, however modern sets tend to use narrow 2.5KHz deviation . If I can find a circuit diagram for these I may see if I can modify the deviation range to go from 2 to 5KHz , but it will do for the moment.

They both have a high quality signal attenuator to allow the output signal to be reduced to a low level , these are graduated in uV. Not sure how accurate the calibration is - the calibration label says it was last calibrated in 1986 ! however it is not critical as I will be tuning for maximum sensitivity rather than trying to perform an accurate measurement.

The signal generators were both manufactured by Pye Dublin - I hadn't been aware that Pye had a factory in Ireland , but it seems that they did have a manufacturing facility in Dublin which closed in the late 1960s.

Using this configuration I have successfully aligned several PMR sets including a Kenwood TK-859
, Kyodo KG107, Tait 2020, etc.

Pye SG5U and Sinadder 3 aligning UHF Tait 2020

The alignment procedure differs depending on the radio being aligned - a search on the Internet should turn up the service manual for most common radios. At a high level the procedure is -

  • connect the signal generator to the antenna input of the radio being aligned. 
  • connect the SINAD meter to the speaker output of the radio. 
  • adjust the frequency of the signal generator until a signal is heard in the receiver
  • set the signal generator to produce a 1KHz tone with 5KHz deviation. 
  • adjust the signal level until 12 dB is indicated  on the SINAD meter
  • adjust the front end of the radio according to the radio's service manual ( using an alignment tool not a screwdriver - the ferrite cores can be fragile) try to get the meter to swing as far to the left as possible. 
  • next reduce the signal level from the signal generator to bring the sinad measurement back to 12 dB and repeat the adjustments. The aim is to achieve 12 dB SINAD for the minimum amount of input signal level. 

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