Everything You Ever Wanted To Know About the JRC NRD 545
One of the main selling points of the JRC NRD 545 - and largest source of comments and complaints -
is the DSP audio filtering.
(But Were Afraid To Ask)
In a nutshell, the JRC NRD 545 is a triple conversion receiver. The first IF is 70.455 MHz, the
second is 455 kHz and the third is 20.22 kHz. The third IF output is fed into an
AK5340 18 bit delta-sigma A/D converter, sampled at 78 kHz as near as I can tell.
The output of the DSP is fed into an AK4310 stereo 16 bit D/A converter, producing the
audio output, with a 39 kHz update rate.
The JRC NRD 545 D/A outputs are routed through a 74HC4052 analog multiplexer, allowing either the D/A
outputs to be sent to the final audio output, or the Wideband FM to be used, which recieves
no additional filtering after the CHE-199 unit.
One of the first complaints is that the JRC NRD 545 455 kHz filtering is inadequate. The filter used is
a CFK455F. The specs I could find for this device (assuming it is made by muRata) are as follows:
3 dB bandwidth: ±4.2 kHz
6 dB bandwidth: ±6 kHz
70 dB bandwidth: ±12 kHz
The concern is that too much signal above 10.11 kHz can get through, causing aliasing issues.
One solution is a
that replaces the filter with an 8 kHz unit, or even a 6 kHz unit.
This of course reduces the maximum audio bandwidth as well.
The JRC NRD 545 has two options for the DSP filtering: "loose" and "sharp". I take a look at the differences
in each set of tests. (Note that there is no apparent difference in AM mode)
I ran a series of experiments on my JRC NRD 545 to measure the actual bandwidth of the 545 in various modes and
filter settings. The results are contained on the series of pages below. I disconnected the
antenna inputs into the 545, to get just static, with no signals. The JRC NRD 545 audio was fed into
a program that I wrote, which can display audio spectrums. While perhaps not the ideal way to
perform these measurements, I believe the general technique does work adequately, in the absence of
owning professional measurement equipment.
For some comparisons, here are some audio spectrum measurements from other radios of mine: