Modern Direct Conversion Transmitters

1. Presented by
Kavyashree B R
[1JB16LDN01]
CMOS RF
Modern Direct-Conversion Transmitters
Under the Guidance of
Pushpalatha .G

2. Modern Direct-Conversion Transmitters
• Figure.1 depicts a common example where ωLO=2ωc.
• A divide-by-2 circuit follows the LO, thereby generating ωLO/2 with quadrature phases.
• This architecture is popular for two reasons:
(1) Injection pulling is greatly reduced
(2) The divider readily provides quadrature phases of the carrier, an otherwise difficult task
• The architecture of Fig. 1 does not entirely eliminate injection pulling.
• Since the PA nonlinearity produces a finite amount of power at the 2nd harmonic of the
carrier, the LO may still be pulled.

3. Figure 1. Use of an LO running at twice the carrier frequency to minimize LO pulling.

4. Modern Direct-Conversion Transmitters
• The principal drawback of the architecture of Fig. 1 stems from the speed required of the
divider.
• Operating at twice the carrier frequency, the divider may become the speed bottleneck of the
entire transceiver.
• Another approach to deriving frequencies is through the use of mixing.
• For example, mixing the outputs of two oscillators operating at ω1 and ω2 can yield ω1+ω2
or ω1-ω2.
• Let us consider the arrangement shown in Fig. 2(a), where the oscillator frequency is divided
by 2 and the two outputs are mixed.
• The result contains components at ω1±ω1/2 with equal magnitudes. The “image” of the other
w.r.t ω1.

5. Figure 2.(a) Mixing of an LO output with half of its frequency, (b) effect of two sidebands on
transmitter output.

6. Modern Direct-Conversion Transmitters
• In a transmitter using such an LO waveform, the upconverter output would contain, with
equal power, the signal spectrum at both carrier frequencies in Fig. 2(b).
• Thus, half of the power delivered to the antenna is wasted.
• Furthermore, the power transmitted at the unwanted carrier frequency corrupts
communication in other channels or bands.
• An alternative method of suppressing the unwanted sideband incorporates “single side band”
(SSB) mixing.
• Based on the trigonometric identity cos ω1t cos ω2t - sin ω1t sin ω2t = cos(ω1+ω2)t and
illustrated in Fig.3(a).
• SSB mixing involves multiplying the quadrature phases of ω1 and ω2 and subtracting the
results.

7. Figure 3. Single-sideband mixer (a) implementation, (b) simplified symbol.

8. Modern Direct-Conversion Transmitters
• We denote an SSB mixer by the abbreviated symbol shown in fig. 3(b).
• Suppose each mixer in fig. 3(a) exhibits third-order nonlinearity in the port sensing A sin ω1t
or A cos ω1t.
• If the nonlinearity is of the form α1x+α3x3 , the output can be expressed as
Vout(t)=(α1Acosω1t + α3A3cos3ω1t)cosω2t − α1Asinω1t + α3A3sin3ω1t sinω2t (1)
= (α1A +
3α3A3
4
) cosω1 cosω2t − (α1A +
3α3A3
4
)sinω1t sinω2t
+
α3A3
4
cos3ω1t cos ω2t +
α3A3
4
sin 3ω1t sinω2t (2)
= (α1A +
3α3A3
4
) cos(ω1 + ω2)t +
α3A3
4
cos(3ω1 − ω2)t . (3)

9. Modern Direct-Conversion Transmitters
• The output spectrum contains a spur at 3ω1-ω2.
• Similarly, with third-order nonlinearity in the mixer ports sensing sin ω2t and cos ω2t, a
component at 3ω2-ω1 arises at the output.
• The overall output spectrum (in the presence of mismatches) is depicted in Fig. 4.
Figure 4. Output spectrum of SSB mixer in the presence of nonlinearity and mismatches.

10. Modern Direct-Conversion Transmitters
• Figure 5. shows a mixer example where the port sensing Vin1 is linear while that driven by
Vin2 is nonlinear.
• the circuit multiplies Vin1 by a square wave toggling between 0 and 1.
• That is, the third harmonic of Vin2 is only one-third of its fundamental, thus producing the
strong spurs in Fig 4.
Figure 5. Simple mixer.

11. Modern Direct-Conversion Transmitters
• It is possible to linearize only one port, thus maintaining a small third harmonic in that port.
• The other is highly nonlinear so as to retain a reasonable gain (or loss).
• The two spurs at 3ω1-ω2 and 3ω2-ω1, only one can be reduced to acceptably low levels
while the other remains only 10 dB (a factor of one-third) below the desired component.
• For use in a direct-conversion TX, the SSB mixer must provide the quadrature phases of the
carrier.
• This is accomplished by noting that sin ω1t cos ω2t+cos ω1t sin ω2t = sin(ω1+ω2)t and
duplicating the SSB mixer as shown in Fig.6.

12. Modern Direct-Conversion Transmitters
Figure 6. SSB mixer providing quadrature outputs.

13. Modern Direct-Conversion Transmitters
• Figure 7. shows a direct-conversion TX with SSB mixing for carrier generation.
• Since the carrier and LO frequencies are sufficiently different, this architecture remains free
from injection pulling.
• While suppressing the carrier sideband at ω1/2, this architecture presents two drawbacks:
(1) The Spurs At 5ω1/2 And Other Harmonic-related Frequencies Prove Troublesome,
(2) The Lo Must Provide Quadrature Phases, A Difficult Issue.

14. Figure 7. Direct-conversion TX using SSB mixing in LO path.

15. THANK YOU