[TangerineSDR] 10 Mhz leakage

[Phil Erickson]

Hi all,

  John has the right approach.

  Bill: I guess I don't understand how a 10 MHz clock can generate a 15 MHz
harmonic, unless some kind of a divide-by-2 happens in the radio to get to
5 MHz and then you are seeing the 3rd harmonic.  Others can add.

73
Phil

On Tue, Jun 2, 2020 at 11:19 AM John Ackermann N8UR via TangerineSDR <
tangerinesdr at lists.tapr.org> wrote:

> 10 MHz leakage is a real problem.  cf my lab/ham shack. :-)
>
> But I'm pleased to say that the current design for the Tangerine CKM
> module will avoid, or at least won't add to, this problem.  We're going
> to directly slave the 122.88 MHz ADC/DAC clock to GPS PPS, with no 10
> MHz oscillator in the mix.  So with good practices applied to the
> digital layout, we shouldn't create any HF spurs in the clock system.
>
> We will have a 10 MHz output available (synthesized in the FPGA) but it
> will be for optional uses and we can make it switchable.
>
> BTW, I don't recall for sure but on the original Hermes board which had
> a 10 MHz TXCO to which the 122.88 MHz was locked, I don't think there
> was any bleed-through noticeable in the receiver.  So it's possible to
> avoid the problem with good design.
>
> The real problem is when the 10 MHz is running around on leaky coax
> cables.  Then it gets into the air and is pretty difficult to suppress.
>  (I found that cheap RG-58 jumper cables running in parallel in my lab
> were porous enough to allow two OCXOs to injection lock -- I ended up
> replacing all the jumpers with double-shielded cables and that problem
> has gone away.)
>
> 73,
> John
> ----
> On 6/2/20 11:04 AM, Engelke, Bill via TangerineSDR wrote:
> > A little discovery that might have a bearing on something in the
> > TangerineSDR…. Or maybe this is just my ignorance at work.
> >
> >
> >
> > We’re putting a lot of emphasis on being able to observe the doppler
> > shift of WWV, often focusing on 10MHz. I have been looking at this using
> > FlexRadio 6600 and Spectrum Lab. In observing the frequency moving
> > around quite a bit (by this I mean 20 to 40 millihertz), I thought,
> > maybe that is the Flex’s TXCO doing its thing. So I got a Leo Bodnar
> > GPSDO and started using this as the Flex’s clock.
> >
> >
> >
> > What I noticed is that even with the antenna grounded and the rig
> > connected to a dummy load, there is a pretty significant (and totally
> > stable) 10 MHz signal shown in Spectrum Lab. I speculate that this is
> > the clock signal finding its way into the front end of the receiver. So,
> > when watching for WWV doppler shift, I see a 10 MHz signal (which is
> > comparable in strength to WWV) superimposed on it.  The same signal can
> > be seen on 15 MHz and 20 MHz – not surprising, since the Leo Bodnar
> > GPSDO provides a square wave output (i.e., rich in harmonics). I
> > recognize that the Flex is not a lab instrument, so I have no grounds on
> > which to complain, but still…
> >
> >
> >
> > I wonder if this will play into the Tangerine SDR?  Are we able to keep
> > RF from the clock from leaking into the front end and overlaying the WWV
> > we’re trying to observe? Do we need to?
> >
> >
> >
> > Any thoughts?     -73- Bill AB4EJ
> >
> >
>
>
> --
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>


-- 
----
Phil Erickson
phil.erickson at gmail.com
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