POWER TO TONE CONVERTER (P2T)
There have long been circuit functions called "Voltage-to-Frequency"
(V2F) Converters. These P2T blocks are essentially equivalent DSP
implementations, except that we are using 20*log10(V) as the control
signal. At first, the DSP-10 P2T used the nickname V2F and this is
still used in much of our documentation. Think of V2F and P2T
as having the same end function.
.
The measurement of Sun or Moon noise at microwave
frequencies can
require delicate peaking of the antenna It is necessary to
get the Sun or Moon within the fractional-dB beamwidth. This addition
can greatly aid in this as it allows one to use their ears as a
measurement tool of the noise level, while freeing the hands and eyes
to peak the antenna, or to operate peaking adjustments. This functions
by producing a tone whose pitch is proportional to the average noise
level, in dB. Since all parameters are adjustable, it can be tailored,
experimentally, to a wide variety of situations.
NOISE OPERATION
ALT-V (or v) turns on the Power-to-Tone (P2T)
function. The tone replaces the normal audio output. Normal control of
the DSP-10 remains available, but you will not hear the results of
control changes, since the tone is being heard. ALT-V (or v) then
toggles the Power-to-Tone function back off.
The P2T tone generation is controlled from a dialog box brought up by
CTRL-V. This box can be opened and closed while the ALT-V P2T is
running. The changes will happen when the box is closed. Inside the
dialog box, you can change the P2T gain (Hz/dB), averaging time, and
base frequency by direct data entry. The box is modal, meaning that no
other functions can change while the box is open, and the display is
not updated, either.
The dialog box allows either noise or signals as a control, with more
about signals below. If noise is selected, the entire crystal filter
bandwidth of about 12 kHz is examined. The power is calculated by
squaring the instantaneous I-F voltage and averaging this with a
decaying exponential (RC) filter. The result is converted to dB and a
gain control is then applied that is calibrated in Hz per dB. This can
range from 15 to 3829 Hz/dB. The averaging time constant (this is the
same value as the RC product for an RC filter) can range 0.01 to 100
seconds. Larger time values are slower, but more accurate.
The base tone is set in the dialog box anywhere from 100 to 999 Hz.
Reasonable starting values are the defaults of Gain=250 Hz/dB, Ave
Time=0.5 sec, Base Tone=450 Hz, and Tone AF Level=60.
SIGNAL OPERATION
If one desires to peak or null a signal, the P2T
function has special provisions. From the CTRL-V dialog box, "Signal
Source" is selected and P2T will be driven by spectral data. At the
bottom of the dialog box, the Low and High frequencies are entered.
Within this band of frequencies, there will generally be multiple
spectral bins. Either the average power over all the bins, or the peak
power in any bin can be used as a data source. This comes about since
the FFT spectral data is equivalent to 512 narrow filters, evenly
spaced. You will not generally get the exact frequencies asked for, as
the FFT bin frequencies are fixed. When you close the box, the actual
frequencies are displayed for a couple of seconds. They should include
your requests, plus a few Hz more. The frequencies for signals are the
band edges, not the bin centers. For instance, there is a bin center at
600 Hz. If you ask for limits of 600 and 600, you will get 595 and 605
Hz, if you are in the 4800 Hz width. These are a half bin (a full bin
is 9.375 Hz wide) above and below the 600 Hz center.
In addition, you can select either Average or Peak. This is meaningful
if the band is wide enough to include more than 1 bin, say 550 to 650
Hz. You can either pick the peak power in that band to drive the tone,
or the average over the entire band. For narrow band signals, the peak
seems to work best. I assume that for signals covering multiple bins,
the average would be better (I haven't had signals to do this with!).
RESET
The reset of the P2T tone is manual only and done
with the
ALT-X key. This can be redone as many times as is needed, and in each
case brings the tone to its base frequency and sets the relative dB
indication (just above the frequency readout) to 0.000. In general, it
may be useful to reset the tone several times if the averaging time is
long. The same applies to relative measurements where the initial zero
value is needed. The reset function does not affect the averaging of
noise or signal data in any way. It only moves the tone and zeros the
"delta-dB."
SUMMARY: ALT-V (or v) Toggles the P2T conversion on and off
CTRL-V (or v) Brings up the dialog box for setting parameters
ALT-X (or x) Resets the tone frequency and relative dB
COMMENTS on P2T OPERATION
If power is less than that associated with
450 Hz, the tone will go lower. However, it does not go below zero,
i.e., power less than that for zero Hz produces no tone at all. Noise
P2T is available in FM, while still doing the S-Meter .001 dB
averaging. Their averaging constants are independent. The S-meter
averaging is set by ALT-F3 and ALT-F4 while the P2T averaging comes
from the dialog box. So, you can peak on the Sun or Moon with the tone
and get a longer averaged reading from the S-meter.
The use of signal measurements in FM would require a strange
application. In that case, the signal data comes from the FM-detected
output and does not represent the RF signal strength, but rather the
amount of deviation. Perhaps, there is a use for this!
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of Contents
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