Signal Generator alignment - UK Vintage Radio Repair and Restoration Discussion Forum

18th Jun 2003, 10:59 am

I have just completed the construction of an RF Signal Generator kit SK-303 from http://xtronics.com -intended for the alignment of vintage radios.

Are there any reasonably priced alternatives to that of using a digital radio readout for the purpose of achieving a fuller range of receiving frequencies to align the signal generator.

The kit has a frequency range of 150KHz-30Mhz

This is very much a novice question.

Thanks.

Duke_Nukem · 18th Jun 2003, 1:13 pm

Assuming you have a working (if inaccurate) radio that covers all of the frequencies of interest then you need to tune the radio to a known accurate frequency - the cheapest route is probably to knock up a crystal controlled oscialltor. e.g. :-

http://www.thevalvepage.com/projects...lk/mastclk.htm

This circuit generates a square wave and hence not only generates the crystal frequency but also multiples of 3, 5, 7, ... etc of that frequency ( " harmonics " ). IMPORTANT : Note the " HCU " in the IC's part number - avoid HCT, ACT, etc. You will need a selection of crystals since each successive harmonic gets quieter. 1MHz and 10MHz crystals are pretty standard (i.e. cheap) and you may even find a 4.433618MHz crystal in the back of a scrap colour telly.

The circuit generates an unmodulated signal so isn't the most easy to detect on a radio, so you could try powering the circuit from the output of an NE555 timer IC (cheap, and should be tons of example circuits on the web) configured to oscillate at, say, 100Hz (keep it failrly low as the oscillator will have a finite start-up time).

TTFN,
Jon

18th Jun 2003, 2:55 pm

I may not have been clear in my question.

What I want to do is to calibrate the signal generator kit at this stage, not radio alignment yet.

Could I just hook up an appropriate/budget frequency counter to the signal generator to calibrate it.?

Thanks again

Norm_Leal · 18th Jun 2003, 3:37 pm

Hi Benton

Connecting a frequency counter to output of your generator would be the best way to calibrate. You could also beat the signal generator to known radio stations.

WWV at 5, 10, 15 mhz are used as frequency standards.

Jon suggested building a crystal oscillator which has harmonics and can be used to calibrate. In each case you would beat the signal generator against known frequency.

Norm

Duke_Nukem · 19th Jun 2003, 1:32 pm

Maybe it was me who wasn't clear ; you apply a known frequency to a radio, you then adjust your generator to so it appears in the same place on the radio, et viola you know the frequency.

Obviously a digital frequency meter is the best solution but I assumed you don't have one or access to one ?

TTFN,
Jon

jim_beacon · 19th Jun 2003, 7:02 pm

Jon,

I would argue that the best way of setting the frequency is the zero beat method, against the standard frequency transmissions, though it requires a lot of skill and patience to get a good number of calibration points, and then to extrapolate between them. As well as a good comms receiver (with BFO) that will go from about 50kHz to 30MHz.

Slightly less accurate, but significantly easier, is to build a crystal calibrator, as you suggested, based around a 1MHz or 10Mhz crystal, and a TTL or CMOS divider, which can be made to give a harmonic rich signal on a number of switched intervals (usually 10kHz, 25kHz and 100kHz), this can be accuratly set by using the beat method against WWV, and then used to calibrtae the generator at convenient points. Maplin used to do a good calibrator kit, but I suppose its gone now, along with most of their useful little kits.

The DFM (Digital Frequency Meter) option is a little less reliable for a couple of reasons, firstly, the output of many generators is to low for a reliable count, unless the have a specific " high level " output intended for driving a counter. Secondly, many simpler generators provide the highest frequency band by using harmonics of the band below, this often leads to a harmonic rich output on all bands, and most counters will not count correctly on much other than a sine wave (Racal counters are particularly prone to this - at work, one of our older types of receiver used a logic ciruit in the (crystal controlled) local oscillator, but provided a high level output for LO frequency checks, unfortunatly, this was harmonic rich, intended to be doubled, and would not give any meaningful result on the counter, in the end we had to build low pass filters to give only the fundamental into the counter - luckily the oscillator ran at 40MHz, so it wasn't too difficult to get rid of the 2nd harmonic)

Alternativly, the frequencies of medium wave transmitters are now maintained to a very high level, and radio 4 on 198kHz is a frequency sub-standard, so beating against these would be more than adequate for most domestic radio work.

Hope this sheds a little more light on the subject.

Jim.

18th Jun 2003, 10:59 am	#1
Brenton_Pettitt Guest Posts: n/a	Signal Generator alignment I have just completed the construction of an RF Signal Generator kit SK-303 from http://xtronics.com -intended for the alignment of vintage radios. Are there any reasonably priced alternatives to that of using a digital radio readout for the purpose of achieving a fuller range of receiving frequencies to align the signal generator. The kit has a frequency range of 150KHz-30Mhz This is very much a novice question. Thanks.

18th Jun 2003, 1:13 pm	#2
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: Signal Generator alignment Assuming you have a working (if inaccurate) radio that covers all of the frequencies of interest then you need to tune the radio to a known accurate frequency - the cheapest route is probably to knock up a crystal controlled oscialltor. e.g. :- http://www.thevalvepage.com/projects...lk/mastclk.htm This circuit generates a square wave and hence not only generates the crystal frequency but also multiples of 3, 5, 7, ... etc of that frequency ( " harmonics " ). IMPORTANT : Note the " HCU " in the IC's part number - avoid HCT, ACT, etc. You will need a selection of crystals since each successive harmonic gets quieter. 1MHz and 10MHz crystals are pretty standard (i.e. cheap) and you may even find a 4.433618MHz crystal in the back of a scrap colour telly. The circuit generates an unmodulated signal so isn't the most easy to detect on a radio, so you could try powering the circuit from the output of an NE555 timer IC (cheap, and should be tons of example circuits on the web) configured to oscillate at, say, 100Hz (keep it failrly low as the oscillator will have a finite start-up time). TTFN, Jon

18th Jun 2003, 2:55 pm	#3
Brenton_Pettitt Guest Posts: n/a	Re: Signal Generator alignment I may not have been clear in my question. What I want to do is to calibrate the signal generator kit at this stage, not radio alignment yet. Could I just hook up an appropriate/budget frequency counter to the signal generator to calibrate it.? Thanks again

18th Jun 2003, 3:37 pm	#4
Norm_Leal Retired Dormant Member Join Date: Jan 2003 Location: Livermore, California, USA. Posts: 139	Re: Signal Generator alignment Hi Benton Connecting a frequency counter to output of your generator would be the best way to calibrate. You could also beat the signal generator to known radio stations. WWV at 5, 10, 15 mhz are used as frequency standards. Jon suggested building a crystal oscillator which has harmonics and can be used to calibrate. In each case you would beat the signal generator against known frequency. Norm

19th Jun 2003, 1:32 pm	#5
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: Signal Generator alignment Maybe it was me who wasn't clear ; you apply a known frequency to a radio, you then adjust your generator to so it appears in the same place on the radio, et viola you know the frequency. Obviously a digital frequency meter is the best solution but I assumed you don't have one or access to one ? TTFN, Jon

19th Jun 2003, 7:02 pm	#6
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: Signal Generator alignment Jon, I would argue that the best way of setting the frequency is the zero beat method, against the standard frequency transmissions, though it requires a lot of skill and patience to get a good number of calibration points, and then to extrapolate between them. As well as a good comms receiver (with BFO) that will go from about 50kHz to 30MHz. Slightly less accurate, but significantly easier, is to build a crystal calibrator, as you suggested, based around a 1MHz or 10Mhz crystal, and a TTL or CMOS divider, which can be made to give a harmonic rich signal on a number of switched intervals (usually 10kHz, 25kHz and 100kHz), this can be accuratly set by using the beat method against WWV, and then used to calibrtae the generator at convenient points. Maplin used to do a good calibrator kit, but I suppose its gone now, along with most of their useful little kits. The DFM (Digital Frequency Meter) option is a little less reliable for a couple of reasons, firstly, the output of many generators is to low for a reliable count, unless the have a specific " high level " output intended for driving a counter. Secondly, many simpler generators provide the highest frequency band by using harmonics of the band below, this often leads to a harmonic rich output on all bands, and most counters will not count correctly on much other than a sine wave (Racal counters are particularly prone to this - at work, one of our older types of receiver used a logic ciruit in the (crystal controlled) local oscillator, but provided a high level output for LO frequency checks, unfortunatly, this was harmonic rich, intended to be doubled, and would not give any meaningful result on the counter, in the end we had to build low pass filters to give only the fundamental into the counter - luckily the oscillator ran at 40MHz, so it wasn't too difficult to get rid of the 2nd harmonic) Alternativly, the frequencies of medium wave transmitters are now maintained to a very high level, and radio 4 on 198kHz is a frequency sub-standard, so beating against these would be more than adequate for most domestic radio work. Hope this sheds a little more light on the subject. Jim.