Spectrum Analyzer - VCO Results
 

I've tried benching up a Minicircuits POS-900W VCO module.

With a Spectrum Analyzer (RBW 1 kHz, VBW 1kHz, peak hold, 100kHz span) I've
been hoping to see a clean waveform that drops to -75dBc at 1,000Hz away, as
specified in the Minicircuits spec. Instead, I have something that looks
like a triangle spreading for MHz!

I've got in such a mess, my brain is now totally confused (not difficult!).
I just don't understand why the trace doesn't look good, compared to my own
discrete design(lower frequency) VCOs. Even with the POS-900W out of circuit
(ie. no PLL), well grounded, and run from batteries, the waveform is still
more-or-less a triangle!

What could I be doing wrong? (My other POS-900W's do the same).
Incidenatlly, I have a screenshot (23Kb) of the display which I can e-mail.

Al.
----
mail[underscore]me[at]freenet[dot]co[dot]uk

What could I be doing wrong? (My other POS-900W's do the same).
Incidenatlly, I have a screenshot (23Kb) of the display which I can
e-mail.

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad. (BOTH the positive AND the
negative leads shielded to ground?)

W4ZCB

What could I be doing wrong? (My other POS-900W's do the same).
Incidenatlly, I have a screenshot (23Kb) of the display which I can
e-mail.

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad. (BOTH the positive AND the
negative leads shielded to ground?)

W4ZCB

"Harold E. Johnson" wrote in message
news:_fwlc.22193$kh4.1279179@attbi_s52...


What could I be doing wrong? (My other POS-900W's do the same).
Incidenatlly, I have a screenshot (23Kb) of the display which I can
e-mail.

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad. (BOTH the positive AND the
negative leads shielded to ground?)

W4ZCB




Thank you for the reply.

But surely if the Minicircuits datasheet says the trace will fall to less
than -75dBC at 1,000Hz away I should see that, or something close to it?
Irrespective of the frequency (800MHz)?

Even if the conections aren't ideal, I'd expect 10dB or so more noise, but
it doesn't change however good/bad it's shielded.

I'm probably doing something really stupid.

Al.
---

"Harold E. Johnson" wrote in message
news:_fwlc.22193$kh4.1279179@attbi_s52...


What could I be doing wrong? (My other POS-900W's do the same).
Incidenatlly, I have a screenshot (23Kb) of the display which I can
e-mail.

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad. (BOTH the positive AND the
negative leads shielded to ground?)

W4ZCB




Thank you for the reply.

But surely if the Minicircuits datasheet says the trace will fall to less
than -75dBC at 1,000Hz away I should see that, or something close to it?
Irrespective of the frequency (800MHz)?

Even if the conections aren't ideal, I'd expect 10dB or so more noise, but
it doesn't change however good/bad it's shielded.

I'm probably doing something really stupid.

Al.
---

Alex Collins wrote:

"Harold E. Johnson" wrote in message

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad.

But surely if the Minicircuits datasheet says the trace will fall to less
than -75dBC at 1,000Hz away I should see that, or something close to it?
Irrespective of the frequency (800MHz)?

Well, Harold is pointing out that noise on the tuning line will
show up as broad phase noise. What are you using to drive the
tuning voltage line?

The old Ramsey FX440 UHF transceiver had a horrific noise floor on the
transmitted audio, since the VCO had about 10MHz/volt sensitivity and
the audio modulation was fed from an ordinary op-amp into the tuning
line. You could be seeing a very similar problem.

Dana

Alex Collins wrote:

"Harold E. Johnson" wrote in message

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad.

But surely if the Minicircuits datasheet says the trace will fall to less
than -75dBC at 1,000Hz away I should see that, or something close to it?
Irrespective of the frequency (800MHz)?

Well, Harold is pointing out that noise on the tuning line will
show up as broad phase noise. What are you using to drive the
tuning voltage line?

The old Ramsey FX440 UHF transceiver had a horrific noise floor on the
transmitted audio, since the VCO had about 10MHz/volt sensitivity and
the audio modulation was fed from an ordinary op-amp into the tuning
line. You could be seeing a very similar problem.

Dana

I'm not terribly familiar with the MiniCircuits VCOs, but I have been
involved with getting the phase noise down to low values on some of
our designs. I can tell you that it's important to have power
supplies be very clean, as well as to have the control voltage very
clean (down into the microvolt region or below) as Harold says.

But from what you wrote, I gather there's a problem even with things
supposedly very clean. BTW, I don't 'zactly understand the "POS-900W
out of circuit
(ie. no PLL)" bit. What are you measuring if the oscillator is out of
the circuit? Or are you saying you're measuring the oscillator "bare"
and not in a feedback loop?

Do you have any other signal you can measure in the same frequency
range? Is your spectrum analyzer up to the task? I'd think your 1kHz
res bandwidth is pretty big to be seeing the noise level at 1kHz
offset...I'd want a res bandwidth at most 1/10th the offset. And it
might be interesting to look with much lower res bandwidth if you can,
to see if there is a particular structure to the sidebands, or if they
are indeed random noise.

Cheers,
Tom


"Alex Collins" wrote in message ...
"Harold E. Johnson" wrote in message
news:_fwlc.22193$kh4.1279179@attbi_s52...


What could I be doing wrong? (My other POS-900W's do the same).
Incidenatlly, I have a screenshot (23Kb) of the display which I can
e-mail.

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad. (BOTH the positive AND the
negative leads shielded to ground?)

W4ZCB




Thank you for the reply.

But surely if the Minicircuits datasheet says the trace will fall to less
than -75dBC at 1,000Hz away I should see that, or something close to it?
Irrespective of the frequency (800MHz)?

Even if the conections aren't ideal, I'd expect 10dB or so more noise, but
it doesn't change however good/bad it's shielded.

I'm probably doing something really stupid.

Al.
---

I'm not terribly familiar with the MiniCircuits VCOs, but I have been
involved with getting the phase noise down to low values on some of
our designs. I can tell you that it's important to have power
supplies be very clean, as well as to have the control voltage very
clean (down into the microvolt region or below) as Harold says.

But from what you wrote, I gather there's a problem even with things
supposedly very clean. BTW, I don't 'zactly understand the "POS-900W
out of circuit
(ie. no PLL)" bit. What are you measuring if the oscillator is out of
the circuit? Or are you saying you're measuring the oscillator "bare"
and not in a feedback loop?

Do you have any other signal you can measure in the same frequency
range? Is your spectrum analyzer up to the task? I'd think your 1kHz
res bandwidth is pretty big to be seeing the noise level at 1kHz
offset...I'd want a res bandwidth at most 1/10th the offset. And it
might be interesting to look with much lower res bandwidth if you can,
to see if there is a particular structure to the sidebands, or if they
are indeed random noise.

Cheers,
Tom


"Alex Collins" wrote in message ...
"Harold E. Johnson" wrote in message
news:_fwlc.22193$kh4.1279179@attbi_s52...


What could I be doing wrong? (My other POS-900W's do the same).
Incidenatlly, I have a screenshot (23Kb) of the display which I can
e-mail.

Well, you don't specify the frequency coverage of your other VCO's, MCL's
have a very high MHz/volt tuning rate and unless the tuning voltage is
absolutely quiet, they'll look pretty bad. (BOTH the positive AND the
negative leads shielded to ground?)

W4ZCB




Thank you for the reply.

But surely if the Minicircuits datasheet says the trace will fall to less
than -75dBC at 1,000Hz away I should see that, or something close to it?
Irrespective of the frequency (800MHz)?

Even if the conections aren't ideal, I'd expect 10dB or so more noise, but
it doesn't change however good/bad it's shielded.

I'm probably doing something really stupid.

Al.
---

My experience with Minicircuits is that they have EXTREME sensitivity to
the AFC line. I'd use a resistor divider from the same battery and a
bypass cap.

Something like a pair of 100K resistors and a 10 uF cap. Naturallly,
ground the cap at the oscillator ground pin.

----------

Also, the Vcc has to be well bypassed AT THE OSCILLATOR.

These things are very sensitive to any change in the VCC voltage. A
battery is often not all that low of an impedance and so some good bypassing
will help.

Finally, how are you taking the signal off to your analyzer?

I haven't used the POS-900 but in general, these critters want a paasive
50 ohm attenuator right on their output to avoid signals sneaking back and
screwing up the oscillator.


Jim Pennell
N6BIU