Low cost signal generator with modulation. Is that a pink elephant?

[CHexclaim]

Hi, I do some vintage radios repairs and restorations and need a low cost RF generator with AM modulated output.

There are many options for dds signal generators, some of them quite good, but cannot find any with AM modulated output.

Appart from the old gear that can be found online, does anybody know of a new, low cost, simple RF generator with AM modulation?

Thanks!!
CH!

[NiHaoMike]

How about $5 for a 3 channel, 150Msps USB module?
https://osmocom.org/projects/osmo-fl2k/wiki
For weakly coupling it to the radio under test, connect one of the outputs through a 75 ohm resistor to a loop of wire.

[MarkF]

If you want to build something with a DDS, you can drive an AD9834 'FS_ADJUST' pin
with the modulation signal while using a differential op-amp on the 'OUT' and 'OUT_B'
output pins.  I built a Function Generator using this method to control the amplitude
without modulation.  However, you can replace the DAC with an op-amp to drive the
modulation (input must be positive half only and the voltage into the DDS 0 to 1V).

Reference attached App Note for controlling the DDS amplitude.

[CHexclaim]

Thank you very much for this information. I will check my cheap generator to see if I can tap into the FSADJUST or similar pin. Clearly it could be done by the software of the generator. I don't understand why they don't do it...

CH!

[tautech]

Hi, I do some vintage radios repairs and restorations and need a low cost RF generator with AM modulated output.

There are many options for dds signal generators, some of them quite good, but cannot find any with AM modulated output.

Maybe try a search for AWG and you'll find most do AM modulation so it just depends of the max frequency you need.
Eg. Siglent's lowest spec model: SDG810
https://int.siglent.com/products-overview/sdg800/

[MarkF]

Here is another App Note for the AD9850 from Analog Devices that may be helpful:

[DaJMasta]

Thank you very much for this information. I will check my cheap generator to see if I can tap into the FSADJUST or similar pin. Clearly it could be done by the software of the generator. I don't understand why they don't do it...

CH!

I've seen this done with DACs in RF signal paths before, some will accept tens of kHz range input signals on their input offset/reference channel, so it can be an easy way to generate a signal.  AM modulation is also one of the simpler schemes to implement, so depending on your power and bandwidth requirements, you may only need a few transistors or opamps and a breadboard.

How inexpensive do you need, though?  Even if they don't advertise modulation capability, it should be an easy add for low end sig gens - the $50 FeelTech FY3200 has an AM output option (I haven't used it, though).

[CHexclaim]

I've been looking for sime time into those chinese generators and in fact I have one which is not the cheapest one, in fact, pretty nice for the price. But, never seen one with output modulation. I will look into the one you mention as is in the price range I'm lookin for.

Nevertheless the idea of using the info supplied in the replies to build the modulator into my existing generator is attractive.. but becomes a new project (another one) in my list!

Thanks, CH!.

[MarkF]

If you are looking at purchasing a FeelTech Signal Generator, you
should be aware of their power supply issues.

Here is a review and one persons power supply replacement:

[CHexclaim]

Mark, thank your for your reply. I'll check into those videos.

By the way, I have I Protek 9204c audio generator that I hacked yesterday to reach 1.3 MHz. I was thinking to add AM modulation to it at least to be able to adjust IF cans and dials in the Broadcast band receivers. It would help a schematic... but couldn't find.

Regards.. CH!

[Mechatrommer]

if you can spend a little bit extra, $100+ Uni-T UTG932 or 962 is another mini (but complete) version alternative, it can easily be made portable (as in bring and operate outdoor kind of portable) by powering from 5V powerbank. since your avatar is GHz RF components, may i also suggest ~$250, Erasynth Micro can also modulate AM/FM from voice, so in emergency you can make distress radio call at any freq 12MHz up to 6GHz. with both units, you will not miss anything between DC to daylight, ymmv.

[CHexclaim]

That Uni-T looks as a great option. But in a first search I couldn't find a seller, only one I think in Australia.

Remember that I live in Uruguay. Buying and importing stuff is not like in the movies.

Thanks. CH!

[Mechatrommer]

That Uni-T looks as a great option. But in a first search I couldn't find a seller, only one I think in Australia.

that is a newly released unit few weeks back, so currently only available in aliexpress from official Uni-T store afaik. maybe it will take time to reach ebay et al..

[CHexclaim]

found it in alliexpress. Thanks.