[hpsdr] Unstable HPSDR?

[Robert McGwier]

I have samples of the following part coming:

http://www.national.com/pf/LM/LMX2531LQ1570E.html#Datasheet

I believe we may then use a much better resonator for the oscillator as 
in much higher Q than we can get with simpler circuits for the VHF 
oscillator and the performance will likely be substantially improved as 
a result.  I am going to try Bartram's suggested modifications to the 
VCO based design with this part.


Bob






Christopher T. Day wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Has anyone further thoughts on this e-mail? If I'm reading it correctly,
> it sounds easy for us to wind up with a Horton receiver that is off
> frequency by a Hertz or two anywhere in the HF bands. Given the flack
> that the SDR-1000 has received about its stability and that I doubt
> Richard Hambly will let us do worse that 10**-10 at 10MHertz on
> Gibraltar, I find this prospect disturbing. Is chasing spurs the wrong
> problem?
>
>
> 	Chris - AE6VK
>
>
> -----Original Message-----
> From: Philip Covington [mailto:p.covington at gmail.com] 
> Sent: Saturday, June 17, 2006 5:51 AM
> To: High Performance Software Defined Radio Discussion List
> Subject: [hpsdr] Fwd: Horton LO - further thoughts
>
> ***** High Performance Software Defined Radio Discussion List *****
>
> <Posted on behalf of Phil VK6APH>
>
> I've been thinking further about the VHF VCO that we may need for
> Horton and have come to the conclusion that a packaged solution may
> not be the best answer.  The reason for this is as follows.
>
> Lets assume that the packed VCO covers 100-200MHz with a control
> voltage range of 0-10v.  This means that for each volt change of the
> control voltage the VCO moves
>
> (200-100)/10 = 10MHz.
>
> OK,  lets say we have 1mV of noise on the control voltage due to earth
> loops, power supply noise etc. This voltage will FM the VCO by
>
> 10,000,000/1000 = 10kHz
>
> Even if we can get the noise down to say 10uV then we still FM at
> 100Hz - not good for CW. In practice we will do better due to the
> divide ratio, but on 10m this is only 4 so we still have 25Hz of  FM.
>
> The way round this is to build our own low noise VCO using a nice high
> Q inductor and 'pre-tune' it by switching capacitors across the
> inductor for each band. Lets take an example, the widest VCO swing is
> on 80m where the VCO goes from 112 to 128MHz, a 16MHz swing.
>
> If we build our VCO to require a 10v control voltage swing to produce
> this 16MHz swing then the same 1mV of noise will produce
>
> 16,000,000/1000*10  = 1.6kHz and our 10uV down to  16Hz.  On 80m we
> divide by 32  so this drops to 0.5Hz.
>
> On 10m we have a VCO swing of  8MHZ and with the same 10v control
> voltage swing we get
>
> 8,000,000/1000*10 = 800Hz for 1mV noise and 8Hz at 10uV, we divide by
> 4 on 10m so this drops to 2Hz by the time we reach  I and Q.
>
> We can improve these figures by using a higher control voltage swing
> but either way this will need a low noise power supply and careful
> attention to screening.
>
> 73's Phil...VK6APH
>   


-- 
AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
Laziness is the number one inspiration for ingenuity.  Guilty as charged!


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