9950	2014-04-25 09:38:40	mmeyer504	Digital Readout Noise Problems
Message	Date	From	Subject
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 problems? Mark WU0L mmeyer504@yahoo.com
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. Ed, N5EM
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, Wes w7zoi
9953	2014-04-25 10:13:24	Jim Strohm	Re: Digital Readout Noise Problems
Mark, 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 ... ;) 73 Jim N6OTQ
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 problematic if 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 paths and 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 stage or 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 via the antenna. The fix for this is coax from the RF sample point and the DFD3 gets placed in 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 and VCC can be problematic if its not properly bypassed and treated just like RF. I've built enough high speed TTL and HCMOS that 4 layer board or a really solid ground and VCC bus is a must 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 using a 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 thinner copper 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 antenna inside the box and also conducted to the VFO that is likely not well isolated from the RF and mixer giving 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 on copper with a lid (in a box) really does better. Shielding is often what separated Collins from the 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. 73 Johan SM6LKM mmeyer504@yahoo.com wrote: > > > 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 > problems? > > Mark > WU0L