Products > Test Equipment

2.5 GHz 12 digit frequency meter - Silicon Chip Australia Dec 2007

<< < (2/2)

Emo:
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 dBm
I would not be surprised if the input level needs to be at 100mV max or less.

Eric

floobydust:
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 the erratas:
"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)"

DMartens:
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.

DMartens:
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.

floobydust:
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 IDSS is in the ballpark. VBB should be around 1.3V
You 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.

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod