Getting started with FM stereo alignments

[bluebelly]

I am 7 years into this hobby of trying to repair old AM-FM table radios from the 1950s and 1960s which are mostly tube stuff. I am now moving on to FM stereo receivers from the 1970's and 1980's. I have learned a lot with your help over the years and am looking for your advice on what would be the best equipment for me to buy for aligning FM stereo since I am new to this. After reading a couple service manuals I realize there are a couple new procedures for these FM stereo units that I did not have to contend with on those older FM radios without stereo.

I currently have a modern Siglent DMM, a 2 year old Siglent 200Mhz oscilloscope, an old B&K frequency counter that looks like it is from 1980. I also have a modern signal generator but it only goes up to 40Mhz. I do have one of those old Heathkit IG-102 signal generators that works surprisingly well but I know it is no match for FM stereo on modern tuners.

So, is there one device that I should consider that will cover all the bases? I have read about the Sencore SG 165 as a solution but have also stumbled across a couple articles about the HP unit.

I don't mind spending a little more to make sure I get something that is relatively user friendly since I am new to this.

Thanks,
BB

 

[Punker X]

Most vintage receivers call for the Sound Technology ST1000 which is a great generator, but may need attention at its age. Panasonic make some great generators. Look for ones that do stereo.

You will also need to think about measuring distortion.

 

[bluebelly]

Just got through reading about the Sound Technology ST 1000 and it looks like something I could handle. Looks like they come up for sale on eBay quite often.
I am interested in also correcting distortion so is there another device that I need to purchase?

Is there a specific Panasonic model that would be good in my situation?

 

[Punker X]

Just got through reading about the Sound Technology ST 1000 and it looks like something I could handle. Looks like they come up for sale on eBay quite often.
I am interested in also correcting distortion so is there another device that I need to purchase?

Is there a specific Panasonic model that would be good in my situation?

There's quite a few different Panasonic models. A search of this forum will turn up something. Was recently discussed and has also been in the past.

 

[PE9ZZ]

It depends on what you find and if it fits your budget. Panasonic made some fine generators, as did Sencore. You could opt for a broadcast transmitter or one of those Si4713 boards. These generate a surprising good MPX signal but getting it to work might be a challenge. A ready-made transmitter might be an option with the tradeoff that it prolly contains a limiter. And there is this spur:

 

[bluebelly]

I have the chance to buy a HP 8657A signal generator and a HP 8904A Multifunction synthesizer for the MPX. Do you think these would be good choices? If so, I still need a distortion analyzer but am unsure what would be a good choice.

Thanks,
BB

 

[PE9ZZ]

Do you think these would be good choices?

Seems a bit overkill. Start with something simple, like that Si4713 transmitter and get the hang of it. Once you are comfortable working with tuners you can always upgrade when you come across something suitable.

 

[k6sti]

If you have an FM signal generator with an external input flat to 53 kHz, you can generate a composite FM signal on your computer and feed it to the generator. This will turn it into a stereo FM generator. All you need is a sound card or chip capable of 192 kilosamples per second with output flat to 53 kHz. Use the .WAV files in the .ZIP file I've provided at the last entry here:

Measurement Utilities

ham-radio.com

The files are sufficient for adjusting tuner response, distortion, and stereo separation. Many signal generators can be adjusted for a distortion level below spec so I wouldn't worry too much about the specified distortion level unless it is above 0.1%. Worst case, the FM modulator might be linearized with simple mods.



This is the spectrum of the composite for a 1 kHz tone. The L+R baseband signal is on the left, the 19 kHz pilot is in the middle, and the L-R signal is on the right.

Brian

 

[Richard P]

I've been quite happy with my Leader 3216. The LSG-216 is an older version but still quite capable. I use a USB audio interface (Focusrite Scarlett 2i2) with REW software for distortion measurements.

 

[Bob@FM]

I've been quite happy with my Leader 3216. The LSG-216 is an older version but still quite capable.

Many of Leader stereo generators are often overlooked, but good quality and usually priced lower than others. The more simple LSG-231 is not their best, though may be OK for tube units and low end receivers. The main drawbacks for the LSG-231 are the FM distortion spec (0.5% - no FM linearization), lack of fine RF output level control, and single fixed RF frequency of 100 Mhz.
The LSG-216 appears to be roughly equal to the Panasonic VP-8174, with FM distortion at 0.1%. These are often not identified on auction sites as FM stereo generators, so can for for less $. Manuals are readily available for Leader stuff to.

One I would avoid, despite the low price, is the very old tube based Heathkit IG-37 and variants. You would likely do better with an FM car transmitter, IMHO

 

[Richard P]

And distortion for the 3216 is less than 0.05% . . .

 

[Bob@FM]

All you need is a sound card or chip capable of 192 kilosamples per second with output flat to 53 kHz.

It's not easy to see if they meet that output requirement.
But the linked MPX file may be useful as way of testing the soundcard's output bandwidth.
Most 192k units have restricted input bandwidth to 20 kHz, at least per the spec sheet. So they may not be easily used in a loopback to test output bandwidth, as the DAC may have wider response than the ADC input.
Some inputs can be extended by special settings. That would imply software control over the bandwidth restricting anti-alias filters (as done in the Scarlett 2i2 3rd gen and up, but only extended to 40 kHz).

Whether the output DAC has flat phase response to 53 kHz is really the question. This spec is not normally detailed for these PC soundcard units.
Maybe dedicated 192 k capable audio DACs are more available with flatness in this range, or more easily modified to remove the output filters. Most audio DACs are USB now, so can be fed with the .wav spdif stream direct from Audacity.
A typical scope screen shot of the popular units that work OK, (and those that don't) could help sort it out - maybe.

A couple of MPX PC software sites supply products that generate 192k streams for actual FM broadcast use.
The comments on support forums show their users also struggle with finding low cost MPX compatible sound cards. Most listed on the MPX software sites are long obsolete (M Audio Audiophile 192). One recommended the ASUS Xonar U5 USB, and that's still available. Amazon reviews say they go belly up pretty quick, but they are kinda cheap now ($59.) Who knows what the actual bandwidth is, though. They may work for radio stations with sharp cutoff 40 kHz filters...

 

[Bob@FM]

And distortion for the 3216 is less than 0.05% . . .

The Leader LG3281 and LG3282 are also very good, they show separation greater than 60 dB, S/N over 90dB, distortion less than 0.01%

 

[Bob@FM]

Just ran a few tests on my Focusrite Scarlett 2i2 (2nd gen.), and it worked great for generating a 1 kHz FM MPX L-R signal.

< edit - updated test results below - PC audio driver was not setup correctly last time >

I verified it's fairly flat as a generator to about 80kHz in 192k 24 bit mode, but has a 1-2 dB flat droop from about 20 kHz to 80 kHz.
The output DAC is filtered based on the sample rate, and mirrors the input response. At lower sample rates, the response cuts off at about 1/2 the sample rate, for alias protection.
The source was measured using white noise from both the ARTA generator and Audacity, then played into a Siglent Scope in FFT mode.

What does this all mean for MPX signal generation?

Generating a 1 kHz composite stereo L-R signal is needs 39 kHz, so a 2i2 can play into an FM capable signal generator for both stereo distortion and separation tests.
The 5,10, and 15 kHz L-R signal play fine too, although they may not work quite well due to the slight droop from the 2i2 output. Other DACs currently being tested may work better.
I would guess the later generation Focusrite 2i2 behave similar, although I have not tested them.
In the future, tests results on this topic will be posted in the DIY sub-group at the tuner group at groups.io
Turns out a few cheap DACs may work better than the 2i2 for this...
Bob