DDS VFO

[K7SGJ]

There was some discussion on another thread about DDS VFOs for use in some of the older hollowstate equipment to replace defunct ones. Someone here expressed some interest in using one as an outboard VFO so as not to have to modify the cosmetics of the radio. I have been thinking about one of these units as a second VFO for some older equipment I have, as well as using it as a piece of test equipment when I don't want to haul out and set up one of the station monitors. Due to the relative stability of these things, they could probably also be used to replace xtals in rigs of that type. (Benton Harbor Lunch Box came to mind) I imagine a few mods for output level and impedance matching might be necessary, but still an intriguing project.

So the other day, I ordered one of the boards from N3ZI. It should be here next week since I think they ship from the left coast. Anyway, I received an email from ZI thanking me for the order, and sent along this link to a Yahoo group. http://groups.yahoo.com/group/N3ZI_kits/ . It appears that the designs I have looked at use the same basic IC for the oscillator. However, they seem to differ in that some use an eeprom, a PIC, etc. to manage the operating parameters. N3ZI also includes eeprom programing info if you want to alter the parameters of the oscilator. The one with the PIC didn't have any info, (I may have overlooked it) but if you are familiar with the PIC instruction set it probably wouldn't be a mystery.

When I get this built and tested, I'll post some pix and whatever I find to be of interest. Also, when I get it working with the hybrid radios, I'll pass along anything special that I stumble upon. If anyone has any experience or knowledge of any of these DDS units, please pass along any information you might have. I know others will find it of interest, as will I.

Details to follow, film at ten.

[N8YX]

Sweet.

I've played around with the NJQRP DDS-60 (which IIRC uses an AD8951) and was beginning to kick around the idea of a dual-synthesizer, external, remote-controllable unit which emulates either an early Kenwood (TS440S, etc) or a K2. Of course, you cannot control such features as IF Shift/slope or perform band changes but QSY and A/B VFO selection is well within the realm of possibility...as is a remote S-meter. One could drive an older SO2R setup with such a construct.

[hareynolds]

I am dusting-off a TS-820 that's been idle for the better part of a decade. In addition, I just won a Drake 2A Receiver on fleaBay.

Both look to be candidates for an external DDS board; to get general coverage on the Drake and as a more-stable frequency source for the Kenwood (probably gonna use it for PSK-31). Consequently I thought I'd bump this thread from late last year.

Has anyone used a DDS on these rigs, or similar (e.g. Drake triple conversion receivers or the Kenwood hybrid transceivers)? What output level is required? That is, will I need to amplify the sine wave? (thought I'd ask first rather than try to figure it out myself)

In addition to the NJQRP DDS-60, I've also seen DDS boards from WA1FFL (Hagerty Radio), N4YG's Drake-specific kit, and of course all the listings on fleaBay (search "AD9851" and stand back).

Some designs go to great lengths to shield the AD9851 and related circuits; others, not so much. Any thoughts on that?

In the Drake, I would plug into a spare crystal socket; could I do that on the Kenwood TS-820, or should I be aiming to replicate a VFO and plug it in the back?

thanks
K5HAR

[NQ6U]

Do any of these DDS units work into the VHF range? I'd love to build a 2m SSB/CW rig one day.

[hareynolds]

Highest freq I've SEEN listed for the AD9851 boards is like 70 or 80 Mhz (I believe that's where the sine wave starts clipping or otherwise deforming, as the AD9851 itself will synthesize a wave up to something north of 150 Mhz.

I don't yet fully understand the frequency limits of these chips; can somebody help? Is there a VHF version of the AD chips? What's the physical limitation?

[N8YX]

HAR - greetings!

I've give you the short answer regarding AD8951 specs then elaborate further later:

Most of the rigs of the later 70's/early 80's whose VFOs operated in the 5MHz region take about 1-1.5v p-p of VFO RF energy. The device in question is capable of providing up to 4v p-p.

Next, the datasheets for the AD chips contain verbiage that the highest frequency you should attempt to generate from them is 1/4 of the clock frequency. (Ex: 120MHz reference = 30MHz DDS output).

There are devices coming on the market which can accept a much higher clock frequency. I'll do some digging.

[wa6mhz]

I also have the N3ZI DDS VFO kit which is assembled and tested. I used a blue/White LCD display after the Green one I also got for it didn't work. Looks like it will need a Buffer amp to make it drive various radios. Very versatile and flexible to use with IFs and muliple bands (like Swans which use different VFO Frequencies for the various bands). Haven't decided how to package it all yet, but it sure works great. Nice to have the slow/fast tuning capability too that the new DDS units have.

[K7SGJ]

I did receive the N3ZI VFO and got it put together. I haven't had enough time to do much more than check it out. It is pretty slick. I plan to test it on my Kenwood TS530, and TS130. Since the VFO plug is the goofy hard to find 262 degree din, I had to order a couple, and just got them wired up last week. Maybe this coming week, I can get the thing on the air. It seems very stable, and I'm curios to see what kind of coverage the receiver will do when going past the normal external vfo output frequency, plus I need to add some switches and other things to replicate the features of the TS240 VFO. The thing has a plethora of features, and it will take awhile to become proficient with each of them. After all, I'm old.

[N8YX]

I'm probably going to fit two of my Astro 103s with a pair each (4 VFOs, anyone?) and all of the R7s that I plan to keep with one. The TR7s and TR5s can make do with the RV75s which have been acquired over the years. It's good that someone is making a high-quality, flexible DDS available.