Flexibility of DDS signal generators - UK Vintage Radio Repair and Restoration Discussion Forum

John_BS · 22nd Jun 2019, 10:41 am

A few years ago I splashed out on a Siglent SDG1020 signal generator (20MHz). It has proved invaluable for AM / FM radio alignment, amongst other things.

- both the AM and FM modulation functions are accurate and low distortion: the AM right up to 99%.

- precise linear IF sweeps with a sync output for triggering a scope.

However, with a bit of cunning you can use these generators up to Band II. E.G.

24MHz n% AM signal: set to 8MHz square wave out with n% mod. Level will be 10dB down (third harmonic).
95MHz sweep +/- 500kHz: set output to 19MHz square wave with +/-100kHz sweep (fifth harmonic). Level will be c. 14dB down.
95MHz 75kHz deviation: as above 19MHz but with 15kHz FM deviation.

I've used this technique up to the 9th harmonic. To use even harmonics you should set the duty cycle of the square wave well off 50%: level of harmonic is less easy to calculate!

John

MrBungle · 22nd Jun 2019, 12:28 pm

Those modern AWGs are excellent. Totally agree. I've got the Rigol version, the DG1022Z which with a little bit of jiggery you can convince it's the 60MHz model instead of 25MHz as shipped

I haven't tried FM on harmonics yet - good idea though that is!

I tend to use mine as an HF signal generator and with the brain transplant it just reaches into 6m on fundamental. Signal quality and phase noise is excellent and output is calibrated in dBm although if you want to go low, an external attenuator or two is required.

Also a good tip: if you build a W7ZOI power meter as mentioned in [1], plug that into a Y channel on your scope and the trig output on the signal generator, set it to sweep a log sweep you can get a log/log plot of filter response with a vertical range of 25mV/1dB. Great for IF, LPF stuff. Here's mine with a QRP transmitter LPF

[1] https://www.qsl.net/sz1a/download/bu...er%20meter.pdf

22nd Jun 2019, 10:41 am	#1
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,780	Flexibility of DDS signal generators A few years ago I splashed out on a Siglent SDG1020 signal generator (20MHz). It has proved invaluable for AM / FM radio alignment, amongst other things. - both the AM and FM modulation functions are accurate and low distortion: the AM right up to 99%. - precise linear IF sweeps with a sync output for triggering a scope. However, with a bit of cunning you can use these generators up to Band II. E.G. 24MHz n% AM signal: set to 8MHz square wave out with n% mod. Level will be 10dB down (third harmonic). 95MHz sweep +/- 500kHz: set output to 19MHz square wave with +/-100kHz sweep (fifth harmonic). Level will be c. 14dB down. 95MHz 75kHz deviation: as above 19MHz but with 15kHz FM deviation. I've used this technique up to the 9th harmonic. To use even harmonics you should set the duty cycle of the square wave well off 50%: level of harmonic is less easy to calculate! John

22nd Jun 2019, 12:28 pm	#2
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Flexibility of DDS signal generators Those modern AWGs are excellent. Totally agree. I've got the Rigol version, the DG1022Z which with a little bit of jiggery you can convince it's the 60MHz model instead of 25MHz as shipped I haven't tried FM on harmonics yet - good idea though that is! I tend to use mine as an HF signal generator and with the brain transplant it just reaches into 6m on fundamental. Signal quality and phase noise is excellent and output is calibrated in dBm although if you want to go low, an external attenuator or two is required. Also a good tip: if you build a W7ZOI power meter as mentioned in [1], plug that into a Y channel on your scope and the trig output on the signal generator, set it to sweep a log sweep you can get a log/log plot of filter response with a vertical range of 25mV/1dB. Great for IF, LPF stuff. Here's mine with a QRP transmitter LPF [1] https://www.qsl.net/sz1a/download/bu...er%20meter.pdf Attached Thumbnails