2.5 GHz 12 digit frequency meter - Silicon Chip Australia Dec 2007 - Page 1
Forum section: electronics / testgear

author: DMartens
author_rank: Contributor
timestamp: November 08, 2024, 12:32:39 pm
content: hi ... assembled the above freq meter ... both PCBs were ordered directly from SC and parts from a mix of the usual parts suppliers ... after power up, the SC welcoming message shows ... and then (sometimes) "0.".Using a NEO M8 GPS module, I generated a 1 MHz signal and fed it into CH1 of a scope and connected CH2 to TP2 which is the PIC crystal frequency divided by 8 ... I then adjusted the PIC crystal until the scope showed no (or minimal) phase shift between both 1 MHz signals ... I repeated this for the 32,768 Hz internal time base crystal ... afterwards I also verified both frequencies with another frequency meter I have and both were spot-on.Last thing to adjust is the Offset Adjust for the input stage around the MC10116 ... the SC article says to adjust the 1K pot meter until the display shows "0" with no input signal connected ... which for me I am unable to do consistently ... sometimes it does, sometimes it doesn't.By feeding that same GPS output signal of (let's say) 1MHz square wave into CHA of the freq meter, the display is showing frequencies anywhere between 2 and 4 MHz.If I read the schematic correctly (I am not very familiar with ECL logic and also not with differential amplifiers), putting a scope on TP6, I should see a 1 MHz square wave but I get a 1 MHz square wave with lots of very short "hair" / noise pulses ... adjusting the 1K pot meter from full CCW to full CW I can minimise the noise but I am not able to fully remove it.I am know the values on the schematic in article Pt1 were changed when Pt2 was published. I have installed the values as per PCB silkscreen).Am I overdriving the input stage by applying the 3V3 GPS signal? Is it related to impedance mismatching? Am I picking up EMC since I have not mounted the PCBs in a housing yet? (I cannot test CH B yet as I am missing one SMD component which is on order).Anyone who also has built this unit who encountered similar issues? Or anyone who can advise how I can troubleshoot?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2024, 03:42:25 pm
content: An ECL input should get away with a much smaller signal (e.g. 100 mV should be plenty). They have the tendency to pic up EMI or oscillate if left open.A lack of termination and thus signal reflections that cause ringing is possible with a relatively long cable and sharp edge square wave as from a CMOS output.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 08, 2024, 04:04:09 pm
content: no offence, but you should share the schematic to help others to help you ....some or many of us dont ses it until you go / search on sillicon ...

author: DMartens
author_rank: Contributor
timestamp: November 08, 2024, 04:30:14 pm
content: Good point and screenshot attached ... I had (wrongly) assumed that only people who have built this unit would be in a position to reply ... but that is of course a silly assumption from my end.

author: nctnico
author_rank: Super Contributor
timestamp: November 08, 2024, 04:43:23 pm
content: I'd start by probing each stage to check at which stage the signal starts to look odd and also measure the voltage between the input of IC 5b to see if there is enough voltage difference to create hysteresis.Maybe this is a power supply issue.

author: Emo
author_rank: Regular Contributor
timestamp: November 08, 2024, 05:14:45 pm
content: Hi,Yes you are definitely overdriving the input. The two protection shottky diodes(1N5711) will start conducting from 250 mV. Try a simple resistor divider on the input with 10-20 times attenuation or if you have SMA attenuators pick one with 10-15 dBmI would not be surprised if the input level needs to be at 100mV max or less.Eric

author: floobydust
author_rank: Super Contributor
timestamp: November 08, 2024, 05:34:56 pm
content: I have this circuit pic from some random site but copyright is 2003. Are you sure it's a 2007 article? ECL input stage resistors are different values from OP's circuit.edit2: SC Oct. 2003 then Feb. 2007 has the "Mk.2" 50MHz Frequency Meter. Same front end and ECL circuit and values. Jaycar or Altronics versions.You can check theerratas:"50MHz Frequency Meter, October 2003: (1) VR2 on both the circuit and overlay should be 10kΩ. The parts list is correct. (11/03) edit: ah it's only for LCD contrast(2) The 470pF capacitor between pin 6 of the LCD and ground may need to be larger in value for the display to operate. A value up to 2.2nF may be required if the display does not show any characters. This value of capacitance may cause the character preceding the word "HIGH" when the Resolution switch is pressed to have a couple of bars instead of a blank space. The normal frequency display when the switch is released will not show any abnormalities. (01/04)"

author: DMartens
author_rank: Contributor
timestamp: November 09, 2024, 07:28:06 am
content: Thanks for the hints.Power supply is 4.98V coming out of the 7805 regulator ... and the scope does not shows excessive ripple or other artefacts.I probed each stage already and it seems problem is already appearing before the JFET, possibly even before the 100k resistor.So the hunt continues.

author: DMartens
author_rank: Contributor
timestamp: November 09, 2024, 08:05:11 am
content: Thanks for the reply floobydust.The input circuit is indeed recurring in many freq meter designs.It can be traced back to the original Motorola App Note AN-581 for their ECL logic families, dating back to the late 1970's.It is used in a 1GHz meter in Silicon Chip Australia of Nov 1987.The 50MHz one you screencapped can be found in Silicon Chip Australia from Oct 2003.As mentioned, also used in the 2.5GHz on in Silicon Chip Australia from Dec 2007.And it was used again in the 55MHz meter in Silicon Chip Australia from Aug 2016.The difference between all the above versions are mostly in the values of resistors and caps: the parallel input cap (between 18pF and 68pF), the values of the resistors on both side of the Offset Adjust pot (1K and 2K2), and the feedback resistors on IC5c Schmitt trigger (100/100ohm or 220/330ohm). The Dec 2007 design seems to deviate the most from all the other ones. Instead of resistors in the hundreds of ohms range, they are now 180 ohm and 1 Kohm. There is also an errata for the Dec 2007 which changes the two 470 ohm resistors to ground on IC5c pins 2 and 3 to 360 ohms.As per my previous suspicion and as noted by Emo/Eric, it seems like overdriving the input circuit is definitely contributing to the problem. I used a resistor divider to bring the input amplitude to <250 mV. Situation improves for lower frequencies (<100 kHz) but not for frequencies between 1 and 10 MHz.(In the meantime, I put the PCBs in a box made with 4 grounded single-sided copper-clad PCBs to shield them from EMI ... but no improvement at all.So need to continue.The specs mention the sensitivity for the Channel A is <20mV below 20MHz, <75mV below 100MHz and <250mV above 100MHz.Was hoping someone who also built the unit may have had similar experiences and was able to resolve them.Or maybe someone who is using it can confirm the meter needs relatively small signals to work properly.

author: floobydust
author_rank: Super Contributor
timestamp: November 09, 2024, 04:43:14 pm
content: It is a build from 15 years ago so it would be hard to find others.The Offset Adjust trimpot I think has side-effects, all amplifier stages are DC-coupled and the Schmitt trigger section will be affected.What does the JFET source voltage measure, input BNC shorted. To see if IDSSis in the ballpark. VBB should be around 1.3VYou can inject a low frequency signal say a hundred kHz and poke around with a scope. It might just have a noise or jabber problem.