[hpsdr] Fwd: Horton LO - further thoughts

[John B. Stephensen]

My experience has been that you must put low-noise filters in the power
supplies at the VCO and any op amps generating the control voltage. Remember
that the voltage references in IC regulators are noise sources so
discrete-component active filters are best. The VCO control voltage must be
as far away as possible from any digital lines and the ground plane under
this area are must be solid.

VCO performance can be optimized by using the largest possible inductors and
the smallest possible tuning range (1 ham band). 1/4" diameter solenoid
inductors work well at 100-150 MHz if they are less than 1/2" long. The
inductor must be located away from digital logic or shielded in a box whose
walls are at least 1 coil diameter away from all edges of the coil.

73,

John
KD6OZH

----- Original Message ----- 
From: "Philip Covington" <p.covington at gmail.com>
To: "High Performance Software Defined Radio Discussion List"
<hpsdr at hpsdr.org>
Sent: Saturday, June 17, 2006 12:50 UTC
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
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