Homemade External VFO

[David McKain]

A friend of mine is proposing to me that I can build an external VFO for
my Kenwood TS-520S.
I was interested since the current VFO doesn't seem to stay stable
(spring tension, warmup) and,
unlike most of the newer HF rigs, my radio is not a general coverage
reciever (1.8,3.5,7,14,21,28,28.5,29.1) +600KHz.
The chip that he suggested is only about $20 and will oscillate from 0
to 40MHz with a sensitivity of
less than 0.006Hz. I would use an RS-232 port to control the chip and
write a small software program
to operate as the VFO controller.

Has anyone had any experience trying to do this?
Any suggestions would be appreciated (except to by another rig)

73
KC8GGV
Dave

[Woody White]

What is the chip number? Sounds like a PLL, not (an analog) VFO. BTW,
do you mean stability rather than sensitivity? - Woody

--
de Woody, WB4QXE, woody_white@worldnet_att_net (_=. No AutoMail)
Work: Electron Microscopist/Microanalysist
Balance: Ham radio "homebrewer", shade tree mechanic,
'90 Nissan 240SX, wish still had my Mcycle too!
http://www.geocities.com/capecanaveral/3722
.

[John Seboldt]

In article <3343B5...@cemr.wvu.edu>, David McKain
<mck...@cemr.wvu.edu> wrote:

> A friend of mine is proposing to me that I can build an external VFO for
> my Kenwood TS-520S.
> I was interested since the current VFO doesn't seem to stay stable
> (spring tension, warmup) and,
> unlike most of the newer HF rigs, my radio is not a general coverage
> reciever (1.8,3.5,7,14,21,28,28.5,29.1) +600KHz.
> The chip that he suggested is only about $20 and will oscillate from 0
> to 40MHz with a sensitivity of
> less than 0.006Hz. I would use an RS-232 port to control the chip and
> write a small software program
> to operate as the VFO controller.
>
> Has anyone had any experience trying to do this?
> Any suggestions would be appreciated (except to by another rig)
>
> 73
> KC8GGV
> Dave

See the R2 RECEIVER thread about DDS VFO problems. You cannot use a DDS
directly for receiving applications; noise and spurious responses are too
high. It has to be cleaned up with a PLL after it.

Now, that can be qualified a little bit. I have a flyer for new, cheap
DDS's from Analog Devices. The AD9850 clocks at 125 MHz, so it should
generate up to 40 MHz or better. Its spectrum has few spurs above the
noise, but the noise in only about 80 dB down. You might get by with that
with an older transceiver like the 520, (maybe it's better than I think,
dynamic range wise), but that's right on the border for any modern design
with a diode ring mixer.

The other problem: you gotta watch out when you stretch the range of a
transceiver by just changing the VFO input. What problems are you going
to get into with the rig's heterodyne scheme? Tuned circuit tuning
ranges? Etc.

John Seboldt K0JD
http://www.pconline.com/~rohrwerk/k0jd/

[Brian Carling (Radio G3XLQ / AF4K)]

There have been some excellent articles on this in QST over the past
few years regarding the DDS type VFO/synthesizer units.

You can build your own or buya kit of parts from Engalnd or from S&S
Engineering in Maryland.

Woody White <"eMail "@ my.www.page.Mailto> eloquently wrote:

>What is the chip number? Sounds like a PLL, not (an analog) VFO. BTW,
>do you mean stability rather than sensitivity? - Woody

>David McKain wrote:
>>
>> A friend of mine is proposing to me that I can build an external VFO for
>> my Kenwood TS-520S.
>> I was interested since the current VFO doesn't seem to stay stable
>> (spring tension, warmup) and,
>> unlike most of the newer HF rigs, my radio is not a general coverage
>> reciever (1.8,3.5,7,14,21,28,28.5,29.1) +600KHz.
>> The chip that he suggested is only about $20 and will oscillate from 0
>> to 40MHz with a sensitivity of
>> less than 0.006Hz. I would use an RS-232 port to control the chip and
>> write a small software program
>> to operate as the VFO controller.
>>
>> Has anyone had any experience trying to do this?
>> Any suggestions would be appreciated (except to by another rig)
>>
>> 73
>> KC8GGV
>> Dave

>--

[John Kolb]

John Seboldt (rohr...@pcOnline.com) wrote:
: In article <3343B5...@cemr.wvu.edu>, David McKain
: <mck...@cemr.wvu.edu> wrote:

: > A friend of mine is proposing to me that I can build an external VFO for
: > my Kenwood TS-520S.

: > The chip that he suggested is only about $20 and will oscillate from 0

: > to 40MHz with a sensitivity of
: > less than 0.006Hz.

Must mean a step size of 0.006 Hz here.

: See the R2 RECEIVER thread about DDS VFO problems. You cannot use a DDS

: directly for receiving applications; noise and spurious responses are too
: high. It has to be cleaned up with a PLL after it.

This sounds like a VFO rplacement for the TS-520, so it's only operating
in the region of 4.9 - 5.5 MHz. Wouldn't a AD9008 be reasonably clean
operated this far below it's clock freq?

John Kolb KK6IL jlk...@cts.com