B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 125; Flight Crew Type 10500
Training / Qualification; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Chart Or Publication; Human Factors
We were cleared the RNAV RNP Z 22 at SAT.  We started the approach at MEDIN. At WAFSY intersection; Approach cancelled our approach clearance due to conflicting traffic and gave us a vector of 030 and told us to expect the Visual Approach to 22.  ATC vectored us around the traffic on final and asked if we had the traffic in sight. We saw the traffic and we were cleared the visual to 22.  I asked the PF if he would like me to extend the centerline of the approach from ZIGDA; the FAF.  He said yes and I complied.  The PF then said he was descending to the FAF altitude of 1700'.  I acknowledged; but something didn't seem right.  At that point; I lost the traffic we were following and started searching for that.  I found the traffic and then tried to sort out what was not right.  I started looking out for obstacles and noticed; while on a 45 degree right dogleg to final; a tower at 12 o'clock.  I told the PF to turn right and he did. As we rolled into the turn we got a 'CAUTION OBSTACLE' alert.  The PF started a go-around and climb.  Once clear of the obstacle; we resumed the visual using the RNAV GP for vertical guidance.  We landed without further event.We were flying this as a visual approach and could have done a more complete brief of obstacles given it was night.  Also; we noted that the obstacle was buried under several converging course lines; so I'm [not?] sure that we would have noticed it even if we had done a full brief.  Perhaps having a longer line leading from the obstacle to the elevation might help.
[Report narrative contained no additional information.]
B737 flight crew reported being cleared for a night visual approach to Runway 22 at SAT and attempting to descend to 1;700 feet prior to the FAF. This resulted in a GPWS 'caution obstacle' for a tower less than 2.5 NM outside the FAF at 1448 feet.