Odd RF fet Motorola SRF7042 - Page 1
Forum section: electronics / rf-microwave

author: pienari
author_rank: Regular Contributor
timestamp: October 29, 2024, 08:08:40 pm
content: I have looking for datasheet for Motorola SRF7042 but not found.Anytone 5189 4m have this RF fet .

author: pienari
author_rank: Regular Contributor
timestamp: November 01, 2024, 06:47:48 am
content: I think common problem with SRF7042 is low tx power and need high bias voltage vs. Other rf fets at same freq range.Datasheet would tell but no one have found it??.

author: ftg
author_rank: Frequent Contributor
timestamp: November 01, 2024, 11:25:17 am
content: That's a surprisingly old transistor in a new product.Motorola's semiconductor division became Freescale in 2004 and that SRF7042 seems to have a datecode from 1996.Sometimes I can find datasheets for the more preproduction or semicustom SRF parts by just swapping MRF for SRF, but no such luck with this one.So you might need a Motorola RF devices databook from the late nineties for a chance to know more about the transistor.But you have confirmed that it's a FET and not a bipolar device?

author: pienari
author_rank: Regular Contributor
timestamp: November 01, 2024, 12:23:24 pm
content: Yes it is a FET.Radio was made in 2016 so Anytone uses nos or used parts.I tested output power to 50ohm dummy load.I set bias to minium 2.6V and power was only 7W, i turned bias to max 4.2V output was 40W at 15V.I measured power with Diamond SX1000 what is calibrated.I just chanced new Motorola RD70HVF1.This was surprice and much better.With minium bias what i measured was only 1.7V and output 7W .So i set bias to max 4.2V output was 60W at 15V.I think safe bias would be 2.6-3.0V and i measured 50W output 15V at 3.0V bias.

author: RFDx
author_rank: Regular Contributor
timestamp: November 02, 2024, 11:21:55 pm
content: Quote from: pienari on November 01, 2024, 12:23:24 pmThe 4m version was sold as a 5/10/25W radio. The radio was reviewed in Practical Wireless magazine where it was established it is a 25W radio despite the generic specs in the manual saying otherwise.https://www.radioworld.co.uk/anytone_at-5189_25_watts_fm_70mhz_4m_mobile_transceiverQuote from: pienari on November 01, 2024, 12:23:24 pmBesides the RD70VHF1 not being an exact replacement, is the "bias voltage" directly applied to the gate of the MOSFET? The transistor should normally work in class-C. With Vg=1.7V that would be the case. With a gate voltage of 4.2V the quiescent drain current for the RD70HVF1 is above 10A which puts the transistor in deep class-A dissipating over 150W as heat.Quote from: pienari on November 01, 2024, 12:23:24 pmEven with Vg=3V (Id~4A) you are burning 60W. It is easy to conclude that the driver stage simply has not enough power to make the final stage output the power you are expecting. Yes, you can raise the power gain by putting the transistor in class-A but that is imo not normal operation as intended by the manufacturer.

author: pienari
author_rank: Regular Contributor
timestamp: November 03, 2024, 10:45:01 am
content: You are right ab. Driver stage i think it wount have enought juice.Driver is RD07MVS1 and i checked schematics voltage is suplied with 6V regulator.That is a bit low.What i have found many of the chinese made 60W Vhf or 40W Uhf radios can be found same boards.AnytoneCRTZodiacV-ComRetevisAlincoI checked Retevis RT-9000D 60W biassing circuit and it was same.Bias was high 4V and tx current was 8.1A at 60W.Edit. For your question.Bias is feeded thru resistor.There is some strange bias compensation to keep the tx output stable different psu voltages.That is a bit dangerous when bias is set at Vss 14.3V bias ab. 3V , Vss 12V bias is risen to over 4V !.