EMRFD Message Archive 9950
Message Date From Subject 9950 2014-04-25 09:38:40 mmeyer504 Digital Readout Noise Problems Today at 5:31 PMI am currently using an Allmost All Digital Electronics DFD3 frequency readout for a homebrew transceiver project.It is working fine for the frequency readout application, but it is introducing birdies and various odd noises in the received signals. If I turn off the DFD3, all the extraneous signals and noises go away. I have used separate batteries to power the DFD3 and the transceiver, but this does not help. I have tried using a fairly large resistor (5k) between the connection to the VFO to isolate the DFD3, but this did not help (any higher resistor and the DFD3 could not read the frequency).If the DFD3 is disconnected from the VFO, but still powered up, there is no problem.The VFO runs at 5.0—5.5MHz.I do not know it the DFD3 is feeding garbage back into the VFO and that is causing the problems, or if it is actually radiating the spurious stuff and the rcvr picking it up.Has anyone had a similar problem? Does anyone have ideas to fix this problems?MarkWU0L 9951 2014-04-25 10:02:00 Ed Manuel Re: Digital Readout Noise Problems Mark,
I would try passing the shielded cable from the DFD3 to the VFO through ferrites on both ends (several turns on a binoculor or several stacked toroids) right at the counter and right at the VFO to see if that solves - or improves the situation. I do not have a DFD3 myself.
9952 2014-04-25 10:12:19 w7zoi Re: Digital Readout Noise Problems Mark, et al,I ran into a similar problem with a PLL. Digital noise from a board with just a little bit of HCTTL logic on it was getting to the VCO and compromising the output spectrum. A common base buffer amplifier cleaned things up. The buffer should NOT be on the same board as the logic, for that would allow the logic ground currents to couple around the amplifier. An example of a common base buffer is in the signal generator in Chapter 7 of EMRFD. Figure 7.27.Good luck.73, Wesw7zoi 9953 2014-04-25 10:13:24 Jim Strohm Re: Digital Readout Noise Problems Jim N6OTQMark,
You might also add a switch or two to the readout to allow it to operate like a "spot" control -- you rarely need to see exactly what frequency you're on -- assuming, of course, that your VFO has an old-timey variable capacitor and a knob like they used "in the olden days." In that case, a dial card would be sufficient, and you'd merely need the DFD3 to check for band edges and for logging, and of course for keeping a sked. Keeping it off except when you need it will eliminate the birdies.
In addition, you said "battery" so I'm thinking that you have a power budget. Keeping the DFD3 turned off except when you need it will let you operate longer.I've found that unless my rig is PLLed to within a few Hz per year, any digital readout is an annoyance most of the time. But that's just me ... ;)
9954 2014-04-25 11:02:37 Ashhar Farhan Re: Digital Readout Noise Problems these are often caused by ground loops as well. it is best to first figure out which line(s) are causing them and then isolate them. often, these are tough to solve unless you move some pieces of circuit around in the layout. it may be too late to do them.a grounded gate or common base buffer amplifier is a far better way to isolate the VFO rather than using a resistor.- f 9955 2014-04-25 16:43:25 kb1gmx Re: Digital Readout Noise Problems This is a common problem especially if the display has LEDs. THis is also problematicif there are LCDs sue to all the high speed switching.There are two paths into the VFO with the noise caused by that, DC and ground pathsand direct to the point of coupling (RF). Running on batteries should this is not the path.Usually the coupling is to a sensitive part of the VFO and really should be to a isolated stageor better using some form of isolation amplifier. The resistor showed it is more likely radiated.The other path is all the high speed switching is radiated and its getting into the RX viathe antenna. The fix for this is coax from the RF sample point and the DFD3 gets placedin a box with coax connector and DC brought in via bypassed feed through capacitors.Allison/kb1gmx 9956 2014-04-25 16:55:02 kb1gmx Re: Digital Readout Noise Problems Wes,As someone that has played with PLLs, bug time yes, the switching noise on the ground andVCC can be problematic if its not properly bypassed and treated just like RF. I've built enoughhigh speed TTL and HCMOS that 4 layer board or a really solid ground and VCC bus is amust especially when the chip count starts to reach more than dozens.However the point of pain is low level circuits seeing the radiated noise. Last PLL built usinga chip (145170) but the VCO, reference and PLL chip were happier (frewer spurs and sidebands)if they were in well shielded enclosures built up on the circuit board. Seems .032 or thinnercopper clad is is really handy for this.It's likely that the real problem with the DFD3 is that the noise is both radiated to the antennainside the box and also conducted to the VFO that is likely not well isolated from the RF and mixergiving it a direct injection point.The give away is power off it stops, separate batteries its still there.This is one of those how its made things that makes a difference and often DEAD bug oncopper with a lid (in a box) really does better. Shielding is often what separated Collins fromthe next cheaper brand..Allison/Kb1GMX 9957 2014-04-25 17:34:41 Sandeep Lohia Re: Digital Readout Noise Problems & if all above fails...
most simple solution = install a 'press to read' PTR
here, I installed a switch ON/OFF @ counter. Hi
learnt that from BEL's delayed-circuit, 150 W MIL RIG
Please encourage recycling, reuse or repairing of E-waste.
░7░3░ ░d░e░ ░V░U░3░S░X░T░
9964 2014-04-27 23:47:32 Johan Bodin Re: Digital Readout Noise Problems I ran into the same problem many years ago with a direct conversion 80m
QRP rig. A strong sharp hum signal was heard when the PIC based counter
was connected to the VFO. Adding isolating buffers between the VFO and
the counter did not help at all. I found that the noise was caused by
the gating circuit in the frequency counter which chopped the VFO signal
at ~100Hz (10ms gating time for 100Hz resolution). The chopped signal
was in fact an AM signal, 100% modulated with the gating square wave,
and this very strong signal was radiated inside the box. The hum
sidebands were perfectly centered around the LO frequency, of course,
and the RX received them as any other AM signal.
I solved the problem by adding a NE612 mixer with a 2MHz crystal, a PNP
buffer and a low Q 1.55MHz tank at the output. This circuit shifted the
3.5-3.6MHz VFO frequency down to 1.5 to 1.6MHz signal for the frequency
counter. I had to subtract the 2MHz offset in the PIC software but then
everything was fine and no "digital noise" was heard in the RX.
> Today at 5:31 PM
> I am currently using an Allmost All Digital Electronics DFD3 frequency
> readout for a homebrew transceiver project.
> It is working fine for the frequency readout application, but it is
> introducing birdies and various odd noises in the received signals. If
> I turn off the DFD3, all the extraneous signals and noises go away. I
> have used separate batteries to power the DFD3 and the transceiver, but
> this does not help. I have tried using a fairly large resistor (5k)
> between the connection to the VFO to isolate the DFD3, but this did not
> help (any higher resistor and the DFD3 could not read the frequency).
> If the DFD3 is disconnected from the VFO, but still powered up, there is
> no problem.
> The VFO runs at 5.0—5.5MHz.
> I do not know it the DFD3 is feeding garbage back into the VFO and that
> is causing the problems, or if it is actually radiating the spurious
> stuff and the rcvr picking it up.
> Has anyone had a similar problem? Does anyone have ideas to fix this