RadioBanter - Switched reactances in VCO

On 08 Nov 2003 23:35:19 GMT, (Avery Fineman)
wrote:

Assuming a VCO control loop bandwidth of 5 KHz, temperature on
the warm side at 320 K, and a resistor value of 10 KOhms, the
RMS noise voltage would be about 0.94 microVolts.

Typically the DC end of the resistor is heavily bypassed to ground,
thus the resistor is more or less in parallel with the resonant
circuit reactances. In many cases a 10 kohm resistor would heavily
load the Q of the resonant circuit and thus, a larger resistor (say 1
Mohm) might have to be used. This would increase the noise voltage ten
times.

Looking first at a free running VCO with the control signal coming
through a large resistor. The noise is generated at an infinite
bandwidth, but the varactor (and other stray) capacitances (often in
the order of 10 pF) in the resonant circuit will form a first order RC
low pass filter with the -3 dB point at 100 kHz .. 1 MHz. Above this
frequency the noise power drops by -3dB/octave, thus it contributes
the same amount of noise power as below the crossover point.

Thus, for a free running VCO, the noise voltage could be up to 10
times larger than previously assumed at 5 kHz bandwidth. With a larger
resistor, the boise voltage would be in an order of 100 uV RMS.

If the VCO is part of a PLL, the only effect is to the noise
frequencies _inside_ the loop bandwidth. The noise outside the loop
bandwidth is _not_ attenuated. The loop bandwidth can be quite narrow
in the order of 100 Hz in some channellised systems especially if a
fixed prescaler is used. The loop bandwidth could be well below 100 Hz
in VHF/UHF FM gears if the audio (and CTCSS) modulation is injected
directly into the PLL control voltage line. Thus, the PLL cleanup
operation affects only a minuscule part of the noise bandwidth.

If the VCO control voltage range is 4 VDC and the tuning range is
approximately linear over 4 MHz, that 0.94 uV will produce a phase
noise of 0.94 Hz RMS. Not a great deal...:-)

At least in many VHF/UHF systems, the VCO sensitivity could be 10
MHz/V and with a noise voltage of 100 uV, the phase noise would be 1
kHz. This would severely limit the SNR obtainable when using FM in
addition to the increased reverse mixing. These kinds of problems are
common with VHF/UHF FM gears.

However, some badly designed simple up converting general coverage HF
receivers could suffer from quite high phase noise, if a single VCO is
used without switching in reactances. In such receivers, any CW signal
would sound as it is coming through the aurora :-).

Clearly, this discussion shows that picking the right initial
assumptions, quite different conclusions can be made.

Paul OH3LWR