V2 RF Frontend Alternatives

There are several alternative Hermes-Lite version 2 frontends being investigated. Some goals of these efforts are:

• Reduce spurious emissions, in particular
• 22.981 MHz spur when transmitting at 24.895 MHz (273.728 - 5Ftx)
• 48.833 MHz spur when transmitting at 24.895 MHz (73.728 - Ftx)
• 7.406 MHz spur when transmitting at 28.010 MHz (273.728 - 5Ftx)
• Provide enough drive (20dBm without 2n2222a preamp, 5 to 10dBm with 2n2222a preamp) for 5W amplifier

This wiki page summarizes results and contains links for further details of the various alternatives.

AD9866 IAMP

IN3OTD has experimented with cleaning up the IAMP spurs by adding a load across the TxDAC output. He and Joe discoverd that if a load is placed on the TxDAC output, the spurs mentioned above are significantly reduced.

Reporter	22.981	48.833	7.406	Power Out	Notes
IN30TD	-68 dBc	-51 dBc		11-13 dBm	33 Ohm across TxDAC
KF7O	-70 dBc	< -85 dBc	-68 dBc	12 dBm	33 Ohm across TxDAC, reconstruction filter included
KF7O	-74 dBc	< -90 dBc	-66 dBc	12.5-13.5 dBm	168 pF across TxDAC, reconstruction filter included, reduction in VHF spurs

Pros

• Simple as makes use of IAMP
• More increments for gain available
• Higest power output from AD9866, so next gain stage can also produce more power

Cons

• Although spurs are below -65 dBc (0.3 mW if amplified to 1 KW funadmanetal output), they are still higher than using the TxDAC
• Gain increments are no longer linear, some steps appear to be missing
• Requires balun
• Not enough power to directly drive final stage of John's QRP PA

Price

• $0.25 for wind your own on binocular core
• $2.00 for Coilcraft or other SMT part

AD9866 TxDAC + 2N2222A PreAmp

IN3OTD has designed various 2n2222a preamps and performed a series of tests. This uses the lower power but cleaner TxDAC output on the AD9866.

Reporter	22.981	48.833	7.406	Power Out	Notes
IN30TD	-75 dBc at worse			20 dBm
JI1UDD	< -80 dBc			18.5 dBm

Pros

• Inexpensive
• Enough power to drive final stage of John's QRP directly
• Clean output

Cons

• TxDAC has limited gain increments
• TxDAC loading is sensitive, and capacitor must be before balun
• May be hard to repeat build

Price

• $0.25 for wind your own on binocular core up to $2.00 for ready made balun
• <$1.00 for transistor and discretes

AD9866 TxDAC + OpAmp PreAmp

N2ADR has designed a new frontend card based on an inexpensivce repurposed video op amp. You can find the design here and more details in this thread

Reporter	22.981	48.833	7.406	Power Out	Notes
N2ADR	-78.1 dBc	< -70 dBc		10.8 dBm

Pros

• Inexpensive
• Clean output
• No balun required
• Isolates AD9866 from load variations
• 50 ohm output impedance

Cons

• TxDAC has limited gain increments
• Not enough power to directly drive final stage of John's QRP PA, intermediate gain stage still required?

Price

• $2.25 for opamp
• <$1.00 for other discretes