On 3/1/07, Tim Ellison <[EMAIL PROTECTED]> wrote:
> Also, don't trust the S meter on the FT-847. If you have calibrated
> using a signal of known strength, the SDR-1000 is much more accurate.
>
If you have an accurate 1uV signal generator, you can test the
absolute sensitivty of the rx and compare to others without regard to
the S-meter indications. Here is an example of how to do this from
the Elecraft site which make a couple of exellent signal generators at
low cost. Set up both rigs as close as possible in configuration,
filters, mode, preamps, attenuators, etc., and you will find out which
has the better signal to noise and sensitivty.
Here are the steps (much of this applies to the Elecraft units, but
can be used with any rig):
Receiver Sensitivity Testing (Using 1 μV Signal)
The XG1 can be used to test absolute receiver sensitivity, providing a
means of comparing various receivers
or transceivers. The instructions below apply to any type of receiver
or transceiver, but we've included
settings for the Elecraft K1 and K2.
1. If you're testing a transceiver, turn its power output level to
minimum as a precaution, and disconnect the
key or keyer and mic. If you're using an ATU, put it into bypass mode
(CAL or CALS on Elecraft tuners).
2. Connect the XG1 to the antenna jack on the receiver (or
transceiver). You can connect the XG1 directly to
receiver or transceiver using a suitable adapter, such as our model BNC-MM .
NOTE: Keep your hands and other objects away from the XG1 during
measurements, as this may affect the
output signal level, especially at the 1-microvolt setting.
3. Turn AGC OFF if possible. (K2, hold PRE+AGC switches. K1, use AGC
menu entry.)
4. Set RF GAIN to maximum.
5. Turn off the attenuator.
6. Turn on the preamp. NOTE: Preamp OFF tests may also be useful.
7. Select CW mode, narrow passband if available ("CW-N" on some rigs).
8. If possible, set the filter bandwidth for about 500 Hz, the
standard for lab receiver tests. (K2, use FLx =
0.70, which corresponds to about 500 Hz. For the K1, use FLx = 500.)
9. Turn off, bypass, or widen the audio filter if applicable. (K2: see
KAF2 or KDSP2 manual.)
10. Connect the probes of a digital multimeter (DMM) across the
speaker terminals. Set the DMM for 2 or 3
volts AC (or RMS) full-scale. (K2: Connect the DMM across the internal
speaker or the external speaker
jack. If you use the headphone jack, don't plug in headphones at the
same time, since this will form voltage
dividers with R35 and R36, lowering the AC voltage reading.)
11. Set the receiver or transceiver for about 7040 kHz.
12. Turn on the XG1 and set it for 1 microvolt output. Replace the
battery if the low-battery LED turns on.
13. Locate the XG1 signal with the receiver. Peak the signal in the
crystal filter passband.
NOTE: BFO settings can affect sensitivity. If the passband peak occurs
at a pitch well outside the expected
range (typically 500-800 Hz), you may need to realign your BFO
settings using an appropriate method (K2:
see application notes on our web site).
14. Set the AF GAIN control fully clockwise (if this results in a
signal that is loud enough to damage the
speaker or cause distortion, use a reduced setting, but make sure you
use the same setting each time).
15. Note the DMM's reading: _______ Vrms (this is the S+N, or
signal+noise reading). NOTE: This value is
useful for comparing overall receiver gain with that of a reference receiver.
16. Turn off the XG1 and note the new reading: _______ Vrms (this is
the N or noise reading).
Signal-to-Noise And MDS Calculations
Using the results from above, you can calculate the signal-to-noise to
noise ratio (S+N/N) at 1
microvolt, and estimate the MDS (minimum discernable signal) as follows:
A. Divide S+N by N; call the resulting ratio R.
B. Take the base-10 logarithm of R ("log" key on most calculators).
C. Multiply the result by 20 to obtain the S+N/N ratio at 1 microvolt, in dB.
D. If the S+N/N is greater than 10 dB, then the MDS is approximately
equal to the result from (C) subtracted from -107 dBm.
Example: DMM readings of 1.0 Vrms (XG1 on), and 0.030 Vrms (XG1 off).
A. R = 1.0/.03 = 33
B. log(30) = 1.52
C. 20 x 1.48 = about 30 dB (this meets the requirement for step D)
D. MDS = -107 dBm - 30 dB = -137 dBm
A K2 should produce a S+N (XG1 on) reading of roughly 0.4-0.8 Vrms,
and more importantly, an MDS of
about -135 dB or better. With an audio filter installed, the S+N and N
readings may be higher, especially if
you've modified the gain settings, but the MDS should be about the
same. Many factors can affect the S+N
and N readings, including whether a KSB2 or KNB2 is installed, how the
40-m band-pass filter and BFOs
are aligned, how L34 is adjusted, and whether the 2nd XFIL
modification has been made (already present in
K2s with S/N 3000 and higher). If the estimated MDS value is lower
than expected, make sure that you have
the preamp on, AGC off, ATU in bypass mode, and RF GAIN set fully clockwise.
A K1 should produce a S+N (XG1 on) reading of roughly 0.1-0.2 Vrms,
and an MDS of about -130 dB.
S-Meter Calibration (Using 50 μV Signal)
50 microvolts is widely used as the standard for an S-meter reading of
"S-9". The manufacturer of your
receiver or transceiver may prefer a different level; consult its
manual. Also note that S-meter readings are
relative. On some transceivers, including the Elecraft K2, turning the
preamp off or the attenuator on will
drop the indicated value. (The K1 adds one bar to the reading if the
attenuator is turned on.)
To align your S-meter at 50 microvolts:
1. Set up the receiver or transceiver as indicated in steps 1-13 on
the previous page. In this case, there's no
need to turn AF GAIN to maximum.
2. Turn the AGC on (K2, hold PRE+AGC switches. K1, use AGC menu entry).
3. Switch the XG1 to its 50-microvolt setting.
4. Adjust the S-meter "scale" value such that the S-meter indicates
S-9. (K2, use CAL S HI function. K1, use
SIG menu entry, "H" parameter. The K1 does not have a labeled S-meter
scale; use 4 bars for S-9.)
5. Turn the XG1 off. The S-meter reading should now drop to
approximately zero (1 bar may be flickering
on/off). If not, adjust the S-meter's "zero" value. (K2, use CAL S LO.
K1, use SIG, "L" parameter.) Repeat
steps 4 and 5 if necessary.
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