15 May 2012

136kHz grabbers and a DC receiver for the band

A list of all the known LF grabbers (on-line to the internet LF receivers) in the world is available at http://icas.to/argo/grabbers-list.htm . A receiver to use with a 136kHz grabber is available as a kit or ready built from http://icas.to/e/ . This is a simple, but effective, direct conversion design which is already in use at several grabbers around the world.


Anonymous said...

I hope these grabbers use the ISDR-138 SDR DC I/Q DC receivers. The design of the ISDR-138 looks pretty good (crystal oscillator, I/Q divider, FST3253 analog switch). But it has some problems...

1. The performance of these DC SDR designs is highly dependent on a good antenna match.
2. Some mismatch effects can be mitigated by using truly differential output amplifiers, the ISDR-138 does not.

3. There can be significant re-radiation through the antenna connection, there is no mitigation for this in the ISDR-138.

The antenna match and re-radiation issues would be nicely handled if this receiver had a tunable input cascode high-isolation antenna pre-amp. A cascode pre-amp may not have the lowest noise performance but I don't think that's a big deal at 138 KHz.

On the other hand, the non-SDR IDC-135 DC receiver looks worse in comparison. First of all, in my opinion the use of an SA612 mixer/osc for anything but experimenting, toys, or as a mixer in a real receiver chain complete with robust AGC - is a crime. The part has really lousy dynamic range. And that's just the beginning.

Also the choice of a TL071 FET Op-Amp is troubling. For the sell price you would think the designer would at least use the higher performance TL05X amps - or even the low noise Op-Amp from NJR used in the ISDR-138 for example. Also the GBW product of the TL071 "may" not be really high enough for the band limited way part is being used. But I'd have to reverse engineer the circuit to be sure.

Yes, I have no doubt these designs work. But there are some chinks in that armor, especially with the IDC-135 receiver.

At $70 USD plus shipping for either one of these simple kits, I think I'd rather put on my tin hat and come up with something a bit different. But if you need something quick and simple - again, these kits will work.

Just my opinion... David WB4ONA

Steve Olney (VK2XV) said...

G'day Roger - you may be interested in my experience with this unit. See


Amongst the posts there are details of modifications I did to the circuit. One of the mods involved switching around two caps on the input filter which the manufacturer at first wrote and told me was not necessary. Later on it became part of the design with no credit to me and presented as just an error on their part... :-)

Steve VK2XV