Design of a Direct DAC To RF Transmitter - Page 1
Forum section: electronics / rf-microwave

author: kml183
author_rank: Contributor
timestamp: October 19, 2022, 04:30:20 am
content: Hi there,I am trying to design for fun, a Direct DAC to RF Transmitter with a similar block diagram to the attached image. I am going to be transmitting on the 10, 20, 40, and 80m bands. I am just looking for a PEP of 1W. The problem I am running into is figuring out what of amplifier to use. I want to use a DAC with an unbuffered current source as they tend to be slightly cheaper and have a lower settling time (under 9ns). I am not sure where to start when it comes to designing an RF network that takes an unbuffered current supply and then amplifies it.Should I just be using an op-amp with a low slew rate?Is there any resource that would be helpful for learning about RF Amplifier design with off the shelf ampsAny help would be appreciated!

author: jonpaul
author_rank: Super Contributor
timestamp: October 19, 2022, 12:59:01 pm
content: Neither solution is optimizedDDS RF generators are MOPA, master oscillator / Power Amplifier. A low level DAC output drives a wideband RF linear amplifier, with 50, 72, or other Zo, and appropriate tuning network to couple the PA to the antennaAs an antenna load can have any phase angle, complex load Zo, an opamp is not suitableA single stage PA should be ample gain and power, BJT, FET or integrated PA readily availableSee ham radio forum and ARRL, QST for construction articlesBon courageJon

author: mawyatt
author_rank: Super Contributor
timestamp: October 19, 2022, 01:00:13 pm
content: We patented (7903016) a similar device we called a Direct Digital to Antenna or DD2A. This concept used a high power/voltage Depletion Mode GaN FET with the Gate grounded, Drain as the output thru RF choke or transformer and Source connected to a series of lower voltage NPN Collectors or NMOS Drains with their Emitter/Sources grounded. The NPN or NMOS devices represented digitally selected current sources, sinking current from the GaN source.This concept worked very well, and later a former colleague utilized the DD2A to replace the amplifiers in a Traveling Wave Amplifier, which became a "Traveling Wave DAC" in a DARPA program called PowerDAC.https://www.militaryaerospace.com/defense-executive/article/16716367/darpa-to-integrate-da-converters-and-amplifiers-to-enhance-power-efficiency-for-ew-sensors-and-communicationsBest,

author: jonpaul
author_rank: Super Contributor
timestamp: October 19, 2022, 01:29:09 pm
content: Mya watt, fine work,1960s..1970s intense phased array radar development, similar to your patent, for antenna modules,OSC..AMP..PH shiftOne PAR surface used thousands of themJon

author: mawyatt
author_rank: Super Contributor
timestamp: October 19, 2022, 01:52:38 pm
content: Yeah, lots of effort over the years for EW and Phased Array Radar, and still going on (another story)!!This DD2A was preceded by a device we called a Gansistor, (patent 7939857), and the overall goal was a completely integrated monolithic device as mentioned in the DARPA note.We had no IRAD funding to develop this, so came in over the weekend and built up a demo version with a GaN device from Cree and Raytheon, using some 2N3904s I had got from Digikey (out of pocket). Made a quick video of the operation with a colleague and presented at a conference, Lockheed Martin took notes and with a former colleague developed the Traveling Wave DAC for the DARPA program PowerDAC. Our company senior management wasn't very bright and certainly had no vision, and we "suffered fools" so to speakAnyway, this went on to a highly successful endeavor unfortunently for others thoBest,

author: kml183
author_rank: Contributor
timestamp: October 21, 2022, 04:08:19 pm
content: Quote from: jonpaul on October 19, 2022, 12:59:01 pmYa I think I was definitely over complicating this. A discrete amplifier would work perfectly here! Thanks I will check out the resources you listed.Quote from: mawyatt on October 19, 2022, 01:00:13 pmVery cool!

author: donmr
author_rank: Regular Contributor
timestamp: October 21, 2022, 08:13:22 pm
content: Most transmitters have some low-pass or band-pass filtering on the output too. This helps to keep unwanted frequencies from getting out. The rules are pretty tight for what is allowed outside the allocated band. These are usually just passive L-C filters.

author: gf
author_rank: Super Contributor
timestamp: October 21, 2022, 08:48:23 pm
content: What kind of modulation do you intend to use? Any kind of (Q)AM requires a linear PA.

author: kml183
author_rank: Contributor
timestamp: October 23, 2022, 03:02:19 am
content: Quote from: gf on October 21, 2022, 08:48:23 pmI was just planning on doing FSK and ASK. I wanted to just play with some of the ham digi modes like RTTY and PSK31. Good point. I am looking into discrete amplifier designs. a CE Class A should work well. I only need 1W of PEP and a gain of maybe 15 dB.Quote from: donmr on October 21, 2022, 08:13:22 pmYa! I think I am going to just use a ceramic LPF. Something likehttps://www.digikey.ca/en/products/detail/mini-circuits/LFCN-95/13927041,but cheaper ha.