Orion MKII Receiver Frequency Response

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w9ac
Posts: 290
Joined: Sun Apr 09, 2017 4:01 pm

Orion MKII Receiver Frequency Response

Postby w9ac » Mon Jan 30, 2023 6:31 pm

Attached are frequency response plots from my ANAN 7000 with ASIO VAC engaged and measured with SpectraPlus-SC software. SpectraPlus-SC has a built-in ASIO driver. Both measurements were taken with a 4096 filter size in LSB mode, 5 kHz receive bandwidth, no weighting, and peak-hold sweep mode (red-colored sweeps), 10 dB/vert. div.

The first plot shows a band noise sweep, the second shows a sweep from an RF signal generator. The frequency response flatness is exceptional, all the way down to 20 Hz.

I've not measured the response from the headphone jack as my current ChannelMaster file bypasses all audio panel connections. It would be an interesting test as a function of headphone loading impedance. The Orion MKII uses 10 uF audio coupling capacitors at the headphone jack, driven from a Lo-Z active source impedance.

According to Sony, the average impedance of Sony MDR-7506 studio headphones over a 20 Hz - 20 kHz response is 63 Ω, but the true impedance is a function of frequency. Assuming 63 Ω, the -3dB turnover point is approximately 250 Hz. However, at least one measurement source shows an average of 80 Ω, with a maximum impedance of 113 Ω at 64 Hz and minima of about 76 Ω at 1 kHz and 7 kHz. This would account for much better real low-end response than Sony's specification sheet suggests.

Long ago, I bridged the existing 10 uF coupling caps at C16 and C17 with 100 uF, bringing the total coupling capacitance to 110 uF. Using the same 63-ohm headphone impedance, we can simply move the decimal point to the left, and we get a -3dB response down to approximately 25 Hz. Probably the only time this matters is for: (i) ESSB and some AM reception; and (ii) use with low impedance headphones (e.g., 8Ω types) that predate the use of neodymium magnets.

By using an ASIO sound interface, the headphone circuit's frequency response is limited by the device's own output circuit topology -- which generally conforms to the highest studio standards.

Paul, W9AC
Attachments
ANAN 7000DLE SpectraPlus-Noise-Unweighted.jpg
ANAN 7000DLE SpectraPlus-Noise-Unweighted.jpg (374.38 KiB) Viewed 2416 times
ANAN 7000DLE SpectraPlus-Sweep-Unweighted.jpg
ANAN 7000DLE SpectraPlus-Sweep-Unweighted.jpg (370.53 KiB) Viewed 2416 times
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: Orion MKII Receiver Frequency Response

Postby w-u-2-o » Mon Jan 30, 2023 6:57 pm

The TX and RX frequency responses of our radios, both RF and AF, are ruler flat from the mic ADC output to the speaker/headphone DAC input. There's no accounting for what happens once it goes analog ;) (However, this applies only to small RF bandwidths, say less than 10KHz)

I looked at this over a year ago when there was a concern posted in the forum about a frequency response problem. The original topic is here: viewtopic.php?f=9&t=3946&p=17530#p16950

I set RX2 to use ADC0 (same as RX1) and disabled RX2 mute during TX so I could independently scale the full duplex receive signal on RX1 and RX2. Thetis was set up with absolutely no processing on TX or RX. Leveler, phase rotator, expander, CFC, both EQ's and COMP all disabled. Transmit passband was 50 to 5000Hz. On receive, no EQ's, no NR or NB. Receive passband was 0 to 5500Hz. I probably should have set AGC to fixed, but I don't think it would have made any difference.

The receive signal was obtained using the coupler located on the output of my amplifier. The amplifier was in standby. The 8000 was set to 25W output, however a quick look at 100W output shows no difference at that power level. I did not want to keep the radio in transmit at 100W for the time it took to obtain the screenshots. I operated into a dummy load.

Using the built-in signal generator in Thetis, one can obtain a much more precise look at the spectral response of the passband using the swept tone mode and peak hold. This provides a much better measurement than using broadband noise. As you can see from the screenshot below, the channel response through both TX and RX is essentially ruler flat.

The Thetis displays show how perfectly flat it is in both TX and RX. If you zoom into single dBs in the Y axis, you can see a very small ripple, but that is inconsequential. Note all of the above was using the low latency filters. The results are the same with the linear phase filters.

Capture3.JPG
Capture3.JPG (616.11 KiB) Viewed 2408 times

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