7000 receiver s meter calibration

WA2DVU
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7000 receiver s meter calibration

Postby WA2DVU » Sat Jun 27, 2020 10:45 pm

I was checking my 7000 and found that the S meter calibration is off. Listed below is what I found. Any info would be appreciated.

Signal Generator output = -73 dBm (50 mV S9) CW.
HP 3336A (-70 dBm + 3 dB pad)

Perseus reading = -73.2 dBm (AM 6 khz

Apache Reading -60 dBm (AM 8 khz)

Perseus reads what sig gen is putting out. Apache is reading +13 dB higher. Why?

73,

Bill WA2DVU - good on QRZ
Cape May, NJ
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w-u-2-o
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Re: 7000 receiver s meter calibration

Postby w-u-2-o » Sat Jun 27, 2020 11:47 pm

Mine was 6dB low: -73dBm in, -79dBm reported. So I ran the level calibration routine and now it's fine. Not terribly impressive, but no worse than any KenYaeWood radio. Input level verified using a modern Agilent analyzer with a recent, professional calibration.

As for why yours is off 13dB, regardless of what your other radio might report, when was the last time your thirty seven year old signal generator was in the cal lab?

If you trust your signal generator just run the level cal in Thetis.
WA2DVU
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Re: 7000 receiver s meter calibration

Postby WA2DVU » Sun Jun 28, 2020 1:21 am

I have several old sig gens (HP and Cushman) and they are all within 2 dB or so. One was at the cal lab a year ago. I did not know there was a level calibration routine. Where may it be? 73,

Bill, WA2DVU

Check my test bench out on QRZ.
WA2DVU
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Re: 7000 receiver s meter calibration

Postby WA2DVU » Sun Jun 28, 2020 11:04 am

Hi Scott,

I found the level calibration routine and took 5 minutes to calibrate the level. The 7000 and Thetis are amazing! My 2- HP 8640B, 2 - 3336B, and Cushman 5100A outputs are within +/- 1 db. Not bad for thirty-seven old gear. :D

Thank you for all of your help with the ANAN Group.

73,

Bill,WA2DVU
Cape May, NJ
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w-u-2-o
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Re: 7000 receiver s meter calibration

Postby w-u-2-o » Sun Jun 28, 2020 1:15 pm

Glad you found it, Bill. I'm still surprised it was off by that much.

Remember that all power levels displayed in PowerSDR and Thetis are referenced to the rear panel connector; the software add/subtracts for the preamp gain and any attenuation you may be using.
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oe3ide
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Re: 7000 receiver s meter calibration

Postby oe3ide » Sun Jun 06, 2021 6:15 pm

Just noticed, that Thetis is showing -115dBm (~S2) with antenna disconnected or on dummyload (on all bands).

Is "level cal" the only way to correct this ? (don't have an signal gen here....)
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w-u-2-o
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Re: 7000 receiver s meter calibration

Postby w-u-2-o » Sun Jun 06, 2021 8:44 pm

The noise floor you are seeing of S2 on the S-meter with no antenna connected is correct. You might be able to get it a little more accurate by performing a proper level calibration with a calibrated external signal generator, but there is nothing grossly wrong with a level of close to S2 with an unconnected input.

On the 8000 I have here, on which I have performed a level cal. using an external signal generator, into a dummy load, using the standard 2.7KHz filter preset, I get -121dbm with the meter on Sig Avg, and around -112dBm on Signal (peak reading). You are probably using the Signal meter mode (peak reading) and S2 = -115dBm (close enough!)

Looking at this a bit more closely, the thermal noise floor is -174dBm/Hz and 2700Hz = 34.3dBHz. This gives a total noise power of -174 + 34.3 = -139.7dBm in a 2.7KHz bandwidth. Using the Sig Avg meter reading from above, that implies a receiver noise figure of about -121 less -139.7 = 18.7dB, which is a little higher you'd expect for a receiver of this type but not entirely unreasonable. Note that much of that noise figure, 7.5dB, is from the pre-amp, which is an LTC6400-14.
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oe3ide
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Re: 7000 receiver s meter calibration

Postby oe3ide » Sun Jun 06, 2021 8:59 pm

Thanks Scott for the explanation.
With Sig avg I get -122dBm, Sig peak 115dBm.
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oe3ide
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Re: 7000 receiver s meter calibration

Postby oe3ide » Sun Jun 06, 2021 9:22 pm

Also interesting:

PA Gemini HF1K on:
gemini_on.jpg
gemini_on.jpg (228.02 KiB) Viewed 4298 times


PA Gemini HF1K off:
gemini_off.jpg
gemini_off.jpg (205.35 KiB) Viewed 4298 times
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w-u-2-o
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Re: 7000 receiver s meter calibration

Postby w-u-2-o » Sun Jun 06, 2021 9:37 pm

Wow, that's some bad switching noise from that amp!
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oe3ide
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Re: 7000 receiver s meter calibration

Postby oe3ide » Mon Jun 07, 2021 6:16 am

I checked all birdies... they are all outside the ham-bands :-)
GW4NNO
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Re: 7000 receiver s meter calibration

Postby GW4NNO » Mon Jun 07, 2021 7:43 am

I'm sure I read somewhere that the calibration should be done with a filter width of 500Hz. This makes a big difference to S meter readings of noise floor etc.
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Re: 7000 receiver s meter calibration

Postby w-u-2-o » Mon Jun 07, 2021 2:41 pm

At the IARU Region 1 Conference in Hungary 1978 the need for a harmonised standard for the "S-meter scale" was expressed and a proposal was accepted for publication in society journals. The essential recommendation was 1 S-point is 6 dB . At the Brighton Conference in 1981 the recommendation was formally adopted as a standard for amateur radio equipment manufacturers. This recommendation is titled "IARU Region 1 Technical Recommendation R.1". Sadly, I can no longer find the original document on the web. If anyone has that document I'd love to have a copy.

The current IARU Region 1 VHF Handbook states in Section 4.1.1:

IARU Region 1 Technical Recommendation S-METER READINGS
BRIGHTON 1981, TORREMOLINOS 1990
Standardisation of S-meter readings
• One S-unit corresponds to a signal level difference of 6 dB,
• On the bands below 30 MHz a meter deviation of S-9 corresponds to an available power of -73
dBm from a continuous wave signal generator connected to the receiver input terminals,
• On the bands above 30 MHz this available power shall be -93 dBm,
• The metering system shall be based on quasi-peak detection with an attack time of 10 msec +/- 2
msec and a decay time constant of at least 500 msec.


The current IARU Region 1 HF Manager's Handbook states in Section 4.1.3:

S-METER STANDARDS
In order to make a uniform reporting system on the amateur bands possible, taking into account the widespread
use of the 'subjective' S-system, taking into account the large deviations between the characteristics of S-meters on
current amateur equipment, the IARU Region 1 recommends the use of the "S"-system for signal strength
reporting on the amateur bands, based on the following standards:
(a) One S-point corresponds to a level difference of 6dB.
(b) On the bands below 30 MHz a meter deviation of S-9 correspond to an available power of a CW signal
generator connected to the receiver input terminals, of -73dBm.
(c) On the bands above 30 MHz this power shall be -93dBm.


Note that although these handbooks are labeled "Region 1" they are the only handbooks and all Region 2 and 3 references point back to them, i.e. there are not separate handbooks for each IARU region.

In no case do they specify the measurement bandwidth, probably because the standard is intended for the measurement of signal power and not noise power.

The good news is that when making signal level reports and the signal is at least 6dB (1 S-unit) above the noise floor the contribution of the noise power will make little difference to the S-meter reading for any typical amateur radio measurement bandwidth, i.e. a bandwidth below 3KHz. For instance, assuming a 2.7KHz measurement bandwidth and a corresponding S-meter noise power measurement of -120dBm, and a signal power measurement of -114dBm, the total power (signal + noise) is -113dBm, only a 1dB difference. Thus measurement bandwidth is not significant when performing a level cal. on our radios. Indeed, most of us will be using signal generators with signal levels substantially above the noise floor. I usually try to calibrate with an input signal level close to -73dBm (S9). In such a case the difference between signal alone and signal+noise is much less than a thousandth of a dB.

In contrast to signal power measurements, where it is only important that the measurement bandwidth be equal to or larger than the bandwidth of the entire signal, when making noise power measurements the measurement bandwidth is critically important if the results are to be compared to other, similar measurements. Thus in industry it is quite common for noise power measurements to be referenced to 1Hz so that the total noise power in any bandwidth can be calculated and receiver performance more easily compared. For example, given the case of a -120dBm S-meter reading with a 2.7KHz measurement bandwidth, this equates to a noise level of -154dBm/Hz (-120 - 10log(2700)).

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