The Nooelec Ham-it-Up Noise Source

 

 Below are pictures from tests made by connecting the noise output of the Nooelec upcoverter to my Rigol DSA815TG via a 20dB attenuator which was needed in order to limit the total power fed into it to less than 2dBm. Power was supplied from a local PC via a USB cable and, apart from one test noted below, the upconverter switch was in the Passthrough postion. Apologies for the poor definition.
 

 

Scan 0-300MHz 

Noise source OFF.

 

 Scan 0-300MHz 

Noise source ON.

 

 Scan 0-30MHz 

Noise source OFF.

 

 Scan 0-30MHz 

Noise source ON.

 

 Scan 0-2MHz 

Noise source OFF.

 

 Scan 0-2MHz 

Noise source ON.

 

 Scan 0-500KHz 

Noise source OFF.

 

 Scan 0-500KHz 

Noise source ON.
 

 Scan 0-200KHz 

Noise source OFF.

 

 Scan 0-200KHz 

Noise source ON.

 

 Scan 0-200KHz 

Noise source ON.

Upconverter ON.

 

 Scan 0-50KHz 

Noise source OFF.

 

 Scan 0-50KHz 

Noise source ON.

 Looking at the results above it's clear that something needs to be done to improve the perfromance if the noise source is to be used at low frequencies. I noticed that with the noise switched off and the upconverter switched on there was no descernable change in the traces, however as you can see in the 200KHz scan, if the upconverter is turned on when the noise source is turned on the position improves slightly. Presumably, if the USB voltage drops slightly, the LF noise pattern changes.

The culprit appears to be the 12 volt power supply circuit. Obviously the avalanche noise from the zener diode is pretty small so that any power supply noise that's about the same order as this will affect the output. Looking at the published circuit. This shows C24 (4.7uF) for input decoupling and C26 (also 4.7uF) for 12 volt output smoothing. Looking at the picture of the corner of the printed circuit board carrying the noise source I can see that the circut is different to that shown in the published data because the DC-DC converter has been changed from an SC4503 to an NCP1403 which has different pinning. The NCP1403 spec calls for a low ESR output capacitor and suggests this to be 33uF. The frequency of oscillations within the NCP1403 depend on a few factors. The value of the choke used to isolate the input and output can typically be say 22uH or 47uH, but the higher the value the lower the oscillation frequency. I checked the frequency of the spikes and the lowest was 6.583KHz with strong harmonics ranging up to 100KHz and more. The theoretical impedance of a 33uF capacitor at 6KHz is about 0.8 ohm and drops to about 0.05 ohm at 100KHz. The question must be.. what value smoothing capacitor is fitted? I've asked the manufacturer and awaiting their reply.....

I received Nooelec's reply in the form of a couple of pictures of the Upconverter which I've added below...

 
 

 Note that the component designations and choice of values differs from earlier published schematics. Specifically "C26", smoothing for the output of the 12 volt power supply chip, is now "C9". The schematic above shows this to be a 1206 size capacitor of 22uF, which is at the low end of the NCP1403 maker's recommended range of values. So why is there severe interference on the scan 0-200KHz for example? One reason is 22uF is too low for this application which requires a very high degree of amplification of a very low level of avalanche noise. Add low level spikes to the noise and this will be faithfully amplified too. That means the 12 volt supply must be very clean and therefore the value of the smoothing capacitor needs to be adequate. The maker suggests 22-47uF but in this application maybe even 47uF is too low?

I removed the circuit board from its case and measured C9 and found it was 10uF and 0.52 ohms. So its got a fairly low ESR but only half the capacity of the designer's intended part. As there are plated-through holes adjacent, and connected, to C9 it was easy to try additional capacitance and observe the effect. 100uF eliminated the spikes but introduced a low frequency noise hump. Anything less failed to clear the lowest frequency spikes, 330uF cleared the 6KHz spike but appeared to upset the operation of the chip by forcing it to oscillate below a couple of KHz. Rather than add a high value I fitted 100uF. The two scans below show before and after adding the extra 100uF capacitor. Click either picture to see a larger detailed view.

 

 

 The pictures show that the noise is cleaned up at the expense of slightly increased amplitude variation, particularly below 10KHz which is anyway not commonly required in radio alignment anyway. Whether operation of the NCP1403 is adversely changed by using 100uF I don't know. The datasheet on the chip doesn't go into any detail on this but perhaps the chip needs to work harder as evidenced by the low frequency humps?

The noise diode is D8 on the layout above. This was reported to be a BZX384-C7V5, but as that type is coded "WD" and the diode does indeed carry 7.5 volts across it, a BZT52-C7V5 is more likely because that is coded "WC". The diode I selected for my home brew noise source was a SOD123 MMSZ5V1T1G (because I had lots left over from a repair job).

 pending

Return to Reception