Scratch-build AM Superhet - Page 1
Forum section: electronics / rf-microwave

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: April 26, 2022, 01:41:47 am
content: My latest scratch build project is to build a simple AM superhet receiver on top of an older radio chassis in which someone removed the old circuit.It has a gang of 0-470 pF Variable capacitors - I'm using two of them.This is the schematic of what I have in mind. I appreciate any comments on anything that stands out as an outright problem.I've not use the SA602A - which is like the NE602. So this is a self-learning moment.Don't worry about the coil values.

author: Circlotron
author_rank: Super Contributor
timestamp: April 26, 2022, 01:58:10 am
content: MOSFET gate looks vulnerable to static.

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: April 28, 2022, 12:28:01 am
content: Protecting the MPF102 gate. Zener's back to back a start?Also, I realized I had a ground missing on the LO circuit.I've numbered the components for reference. Note: X1 is a ceramic resonator.Here is new drawing:

author: A.Z.
author_rank: Super Contributor
timestamp: May 01, 2022, 08:08:13 am
content: change the input stage to a common gate, that will give you a better match for low impedance antennas, move the diodes between the antenna and the input capacitor, not after it, optionally consider adding a low pass filter between the antenna and the JFET stagesee here for some ideashttps://www.eevblog.com/forum/rf-microwave/common-gate-wideband-rf-amplifiers/and here's another approach which offers both a preamplifier and a preselector stagehttp://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb2018/Steber.pdfyou may even combine the two ideas, placing a low/unity gain common gate wideband stage at the antenna input and following it with a tuned preamp offering adjustable gain

author: CJay
author_rank: Super Contributor
timestamp: May 01, 2022, 12:51:25 pm
content: Change the zeners to a pair of 1N4148 connected inverse parallel, use an antenna coupling transformer something like the KANK3334R (Toko no longer manufacture them but there are NOS and modern equivalents available)Take a look around the web, something like the GQRP Sudden RX project kit will give you ideas, ARRL handbook is useful but if all you want is BCB AM then a lot of the ham projects just add layers of complexity which aren't useful for your needs right now but may be useful later if you decide to 'do more'Ignore part numbers (schematic fragment borrowed from a CB Radio schematic)

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: May 01, 2022, 01:44:19 pm
content: Common gate, hmm. I would have never thought of that.

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: May 01, 2022, 01:45:28 pm
content: 1N4148, ok - that's good. Thanks.

author: A.Z.
author_rank: Super Contributor
timestamp: May 01, 2022, 05:32:31 pm
content: Quote from: Daniel Bingamon on May 01, 2022, 01:44:19 pmare you kidding or what ?anyhow, have a look at those designs

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: May 01, 2022, 06:55:11 pm
content: Seriously, I haven't done much with common gate before.This is more of my hobby. My day job is programming.

author: A.Z.
author_rank: Super Contributor
timestamp: May 02, 2022, 08:46:16 am
content: Quote from: Daniel Bingamon on May 01, 2022, 06:55:11 pmI see, ok.As for the protection diodes, see the notes herehttp://www.kk5jy.net/rf-clipper/As for the common gate stage, it's a quite flexible circuit, especially if used as a "buffer" to match a low impedance antenna to the high impedance input of a receiver, I recommend you to check the two papers (links) I posted, since they may give you some ideas about designing your receiver input stage; as I wrote, an idea may be using a very low gain common gate stage right at the antenna input and then follow it with a tuned preamp offering adjustable gain and, from there, to the receiver stages; such an approach will allow you to have a "preselector" stage which will improve your receiver performance at the cost of a slightly additional complexityAnother approach to the input stage may be using an AD605 IC, either used with a varactor tuned stage (preselector) or as a wideband preamp, if you look at the datasheethttps://www.analog.com/media/en/technical-documentation/data-sheets/ad605.pdfyou'll see that the AD605 offers a -14dB to +24dB range, by adding a -10dB attenuator you'll be able change such a range to a -24dB to +24dB one, the gain/attenuation may either be controlled manually (a potentiometer) or you may control it using an AGC circuit or use a mixed setup offering both AGC and manual setting (to adjust the max gain)

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: May 02, 2022, 11:09:47 am
content: I'll have to make a filter for local stations. One of which is WLW 50kW only 3 miles away by air.

author: A.Z.
author_rank: Super Contributor
timestamp: May 02, 2022, 02:06:09 pm
content: Quote from: Daniel Bingamon on May 02, 2022, 11:09:47 aminstead of a filter, you may have a decent preselector at the antenna input; please, reread the thread (and the various links)

author: Circlotron
author_rank: Super Contributor
timestamp: May 08, 2022, 11:53:34 am
content: C12 and D3 in series is an issue.The capacitor will charge to a certain DC voltage and then that detector circuit will stop working.Also for the audio amplifier there needs to be a capacitor of say 1000uF between the LM386 and the speaker and a small capacitor between the volume pot wiper and the amplifier input.

author: A.Z.
author_rank: Super Contributor
timestamp: May 08, 2022, 04:09:05 pm
content: Quote from: Circlotron on May 08, 2022, 11:53:34 amright, and there are some other spots which will probably need to be reviewed; as is, the circuit seems a straight copy/paste of several different circuits w/o adapting each other; good for brainstorming, but a minefield if one will try it as is

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: May 08, 2022, 10:30:56 pm
content: Yes, some copy/pasting. I've also guesstimated some of the values.The LM386 is pretty basic, I've made those before. The 2N5308 is out of a Heathkit SW-717. I do want to put a capacitor in front of the LM386 somewhere to get rid of the RF.Maybe a resistor after C12 to keep any charge bled off.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: May 27, 2022, 02:14:40 am
content: Two C12s?Jeff

author: vk6zgo
author_rank: Super Contributor
timestamp: May 27, 2022, 03:57:25 am
content: Quote from: Daniel Bingamon on May 02, 2022, 11:09:47 amShift the first LC tuned circuit to the antenna side of the MPF102, otherwise your FET is wide open to powerful signals of any frequency, which will mix & give you unwanted products within your tuning range.Try to look at some old tube type receiver designs, they did things for good reasons.I used to work at a combined AM Broadcast & HF radio Comms station, in the lunchroom of the separate Comms building, we had an old domestic mantel radio.It could receive other AM stations, while being, maybe 300metres away from the "Dual Mast" which was radiating 55kW from one station, & 10kW from another.PS: You will get mixing products from D4 & D5, too!

author: Daniel Bingamon
author_rank: Regular Contributor
timestamp: May 27, 2022, 11:09:37 am
content: "PS: You will get mixing products from D4 & D5, too!"So, voltages on the line could cause the diodes to act like capacitors?I saw a radio on youtube that used ordinary diodes to tune with.This mixer circuit is out of my comfort zone, I may try something more basic.

author: Circlotron
author_rank: Super Contributor
timestamp: May 27, 2022, 12:08:21 pm
content: Quote from: Daniel Bingamon on May 27, 2022, 11:09:37 amThe diodes will act like a mixer.https://en.wikipedia.org/wiki/Frequency_mixer#Diode