Saturday, 14 February 2015

More Rx Bandwidth Measurement

Way back in 2011 I published a design for an analog AF CW filter in SPRAT 146...

The filter was slightly unusual (i.e. it was differentiated from the thousands of other CW filters that already litter the literature) in that it used a gyrator.

It was designed to offer the user a single bandwidth control, which adjusted only the receive bandwidth without changing centre frequency, gain etc.. This (as those of you who play with filters will realise) isn't so easy to achieve, as all the parameters of a filter are interconnected, such that adjusting just one is not as easy as it might sound.

The filter works FB - as measurements of its frequency response, included in the original SPRAT article, testify. The magnitude response at a number of (arbitrary) positions of the "bandwidth" control is shown here...

The filter first was used with my Funster Plus rig, was featured in the Occam's Microcontroller" rig and was reprised in the "Occam's Dagger" rig, where it turned the otherwise rather wide open "d.c. to daylight" frequency response of the Sudden-derived receiver into a more practical proposition.

Well - the Sudden Rx shield is receiving renewed interest, as is the Occam's Dagger rig (a result of my recent demonstration that the Sudden Rx and Tx shields will run FB on the new prototype Si5351 shield). PLUS I have recently demonstrated a technique to directly measure the overall receiving response of a receiver, from RF to AF.

So, with this background and renewed interest, it seemed like a good excuse to look again at the response of the m0xpd CW filter in-vivo.

Here's the measurement set-up on the bench...

As you see, I am using the same arrangement as before. An Arduino (MEGA) generates an RF sweep from a Kanga m0xpd DDS shield, which is passed via an attenuator to the antenna input of the Radio under Test - in this case Occam's Dagger with the m0xpd CW Filter. You can just see the Rx Bandwidth control to the left of the Occam's Dagger rig.

The AF output from the rig is tapped at the speaker output and the (entire) AF signal is detected by the Bruel and Kjær electronic voltmeter, operating as an RMS to dc converter. The resulting dc level is fed back to the Arduino MEGA, which passes the value (along with the frequency to which it is associated) back to a Processing sketch, running on the PC, which graphs the result in real time.

As a base-line, the "native" response of the Occam's Dagger rig is seen in this response measurement with the CW Filter in bypass mode...

As I have explained previously, the abscissa of the graph above describes a 10 kHz RF sweep (from 7.03 to 7.02 MHz). The receiver was operating in lower sideband mode and was tuned to 7.03 MHz, so this will map to an AUDIO frequency range of 0 to 10 kHz.

As you can see from the above, the Occam's Dagger rig can be tuned at 7.030 MHz, and yet I can hear several kHz-worth of CW signals all at once (despite the low-pass slope, which does little to block big gun signals). There is essentially zero selectivity, save that of my ear/brain's ability to discriminate CW signals at different pitches!

Switching in the CW filter and advancing the bandwidth control to the tightest (practical) bandwidth produces the measured result shown in the graphic below...

The bandwidth control is seen to change the response dramatically, giving selectivity (indeed, too much selectivity sometimes, at such an extreme setting).

In fact, the change is continuous, between the limits of the graphic above. Some sense of this continuous change is communicated by the following sequence of measurements, each made at advancing positions of the Bandwidth control (reading down columns, then from left to right)...

Actually, the Bandwidth control can be moved to settings giving even tighter bandwidth - but these are achieved at the expense of increased noise and so are not used. Such a setting is shown in the measurement below, in which both the high Q-factor of the peak and the noise are visible.

So, just as was seen for the software defined radio (with its programmable Rx bandwidth) and for the Parallel IF radio (with its switching Rx bandwidth), this DC receiver is proven to have variable receive bandwidth as measured directly from RF input to total AF output.

I love this method for its speed, simplicity and honesty.

...-.- de m0xpd

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