DDS Signal Generator

[HowardB]

I'm looking at buying a fairly cheap Signal Generator.
Currently looking at the FY6800 60MHz DDS.
Would you recommend this for general radio alignment work?

HowardB

[Herald1360]

Apart from a lot of gripes about driving it from a PC and getting decent arbitrary waveforms out of it most reviews say it's wonderful and a steal at the price. 10.7 MHz sweeping is mentioned as a capability for FM IF alignment too.

For basic general purpose radio alignment it's probably fine, mention of the plastic case and no metal case version suggests that signal radiation may be a problem if you want to measure communications receiver spec limits but there's no doubt there's plenty of bang for your buck.

[HowardB]

Thanks Chris, that was the final endorsement I was looking for.
I've already seen a number of positive video reviews so I think I'll buy one.

HowardB

[Electronpusher0]

Take a look at this thread:
https://www.vintage-radio.net/forum/...d.php?t=163177

I found that these plastic DDS generators radiate far too much from the plastic case. In the above thread I described how I shielded mine with Nickel spray.

Peter

[HowardB]

I'm having second thoughts after reading your thread - but too late, I already ordered one!

Was the Nickel spray effective? If so, where did you get it from? - I can see it on eBay but ridiculously expensive and only available from the US.

In any case, it sounds like a messy business, is there another way of shielding the box?

Were you able to use the DDS sig gen without the shielding to align domestic radios, or is it totally useless for this?

HowardB

[Radio Wrangler]

Make a metal box.

Keep apertures in it small.

Use a bulkhead connector so the rf output coax is hard-grounded as it leaves the box

Decouple the power cable right where it enters the box... capacitors and ferrite beads.

Make some well-shielded coax attenuators so you can drop the level as low as you need. Attenuators with RF connectors can be stacked to add their attenuation (in dB)

Screening is everything!

David

[HowardB]

David, can you please say a bit more about attenuation, what level of signal would you apply to the RF or IF stages of a radio? The generator output can be set from 1mV to 5v in 1mV steps, so is this level too high, are we really talking uV and not mV?

HowardB

[Radio Wrangler]

On some occasions you'll want to go down to the microvolt level and maybe lower.

A common thing is having to sort a receiver that's 'a bit deaf' total failures are easy, but with something a bit off song you'll want to see how much off proper performance it is and be able to monitor the effects of things youtry.

Even doing a simple alignment, you want to do it at low levels so the AGC action doesn't spoil your appreciation of the changes your adjustments make. How do you determine where the peak is on an adjustment if the set changes gain to try to flatten the level?

Making a signal is cheap. Making a stable, controllable frequency is now cheap. But the screening needed to work with sensitive receivers is still very expensive in either time or money.

David

[Electronpusher0]

Originally Posted by HowardB

I'm having second thoughts after reading your thread - but too late, I already ordered one!

Was the Nickel spray effective? If so, where did you get it from? - I can see it on eBay but ridiculously expensive and only available from the US.

In any case, it sounds like a messy business, is there another way of shielding the box?

Were you able to use the DDS sig gen without the shielding to align domestic radios, or is it totally useless for this?

HowardB

I got the Nickel spray from RS some years ago so I do not know the cost, sorry.
Yes it is very effective, the challenge as I said in the other thread is to make sure you mask the areas where the pcb touches the case otherwise you risk shorting it out.

The other option of a larger metal box with minimum apertures is another (and probably more effective) way to go.

Peter

[HowardB]

Do you think Conductive Alumiminium Tape (RS Stock No. 185-9047) would be any good?
It's certainly cheaper than copper tape or the Nickel spray and looks like it might be easy to apply where you want it. But would every piece need to be tied together and grounded?

HowardB

[Radio Wrangler]

A very poor second, I'm afraid. Unless the tapes are seam-soldered, you wind up with slots and leakage from them. You'd find a metal box a lot less work.

I do sometimes use copper tape backed up by soldering it all together to bodge screening for EMC tests. rather expensive nowadays. One thing i used to stock up on at amateur radio rallies were diecast boxes.

David

[Wendymott]

I buy scrap or offcut pcb material for screening.... relatively stiff support... easily cut..just a "****" to unsolder if you make a mistake...... Some of the copperclad Phenolic material is use once only as the adhesive can let go... but rather than EX biscuit tin plate..

[Wendymott]

Added photo's to show screening.... sorry second photo is pre screening

How about a biscuit tin with a bulkhead BNC and a filtered IEC (one of those tin box types) inlet? Might be a bit of a pain to use but cheap.

[HowardB]

OK, the FY6800 has now been delivered and I'm starting to play with it. People have commented that the plastic case will cause it to radiate. I have a MW radio about 1 metre away but cannot detect any interference from the signal generator.

Maybe I'm not understanding the issue properly - does the instrument need to be set to a particular frequency or modulation scheme or connected to the radio for this to be a significant problem?

HowardB

[Electronpusher0]

You may just be lucky, On mine I was using it to test an audio amplifier and scoping the input and output and the scope leads were picking up HF mush.

Peter

[Herald1360]

A MW radio just standing nearby would likely need mV level signals to be affected much. Screening most likely will be needed for uV level signals used for sensitivity measurements.

For domestic radio alignment etc it'll probably be just fine.